The IGBT Protection

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Principle of operation

The first IGBT thyristors were modified to be able to turn on with a voltage command. Consider the BJT equivalent circuit of a thyristor and a MOSFET transistor is added between the collector and emitter of the NPN (lower transistor). The gate of the MOSFET becomes in this case the the IGBT gate, while what was the gate of the thyristor it is no longer used.

By applying an external voltage between the gate and cathode triggers the thyristor, as it is invoked by the PNP current, which in turn sends a current into the base of NPN and the whole is brought to self-sustaining (latching). The latching, normal for a thyristor, Here it is unwanted, because you would want to control the IGBT both upon start off, by simply removing voltage to the gate, as in a MOSFET. It has been tried to make the NPN transistor with amplification characteristics in current (hfe) deliberately shoddy, so as to make it difficult to latching. For shoddy means a much lower gain 1, the lowest possible: despite this measure, it often happened that the latching occurs anyway. In particular, It was frequent in harsh conditions, such as a short circuit: the component remained triggered without the possibility of turning it off, and that meant its destruction.

Arrangement 2 BJT NPN e PNP all’interno dell’igbt (equivalent connection to a thyristor)

It is then removed of all the NPN transistors, by becoming the IGBT, as it is currently, a Darlington configuration between a PNP and a MOSFET. The device that is obtained has the great speed of typical switching MOSFET and the low losses of BJT run.

Schematic of the evolution of the IGBT its graphic symbol

To bring in conduction the IGBT is necessary to apply and maintain a voltage between the gate and cathode, and remove it for shutdown, how to command a transistor: However, here the pilot is in tension. The three external terminals were renamed, for similarity to a BJT: the cathode in emitter, while the anode has become the collector. The gate It remained that, but internally it is connected differently.

In the following, we will analyze the phenomena that occur during the transitional power-on and off of the IGBT: the study is valid for both a chopper configuration or an inverter leg, with inductive load. During such events has:

  • Turn-on: power on (instantaneous overcurrent problems)

  • Turn-off off (overvoltage problems, phenomenon of the current queue).

The waveforms relating to the switching of the IGBT of an inductive load are schematized in the following way.

Schematization transient ignition (turn-on) and off (turn-off) dell’igbt.

Piloting the Gate

It is said that the application of a simple voltage on the gate controls the conduction of the component, but this is true solo a regime. They are in fact inevitably present of parasitic capacitance between the gate and collector and emitter: it is inevitably isolated by an oxide layer from these other two terminals. Then, every time you give a command, you have to fill or empty these capabilities: if I want to do quickly, to minimize the switching time and associated losses, It must be used strong currents, even of the order of 10 A. To send these currents at the gate, also only limited to the switching intervals, it employs a pair of transistors BJT in a complementary symmetry (NPN + PNP), Robust cuts. Giving the gate drive in a manner not decided means creating problems, especially when you want to control the interdiction of the IGBT. Consider L IGBT inserted in a leg of a classic single-phase inverter bridge. Recalling little current from the gate (is: 100 ma), the IGBT, however, tends to interdirsi, and the load current, which is ohmic-inductive, He must go for strength in the upper free-wheeling diode.

such diode, though not instantly, moves to run fast enough (It is of the fast type). What is worrying is the high voltage derivative of (dv/dt) imposed by this recirculation current: it does pass into the parasitic capacitance CGC (gate-collector) a current equal to that relied on by gate.

This current flowing through the parasitic capacitance goes to drive the IGBT, maintaining a constant voltage derived, with a elongation considerable switching time.

Piloting the Gate and parasitic parameters

It must also take into account that, in addition to the parasitic capacitances, They are also present parasitic inductances in the gate control circuit, able to resonate with the capabilities and trigger fluctuations in the voltage on the gate. These oscillations, albeit muted, They may have high peaks able to reignite the IGBT, so that it turns off in hiccups, with great increase in switching losses. The other way around, to the ignition command, the oscillations may submit the gate higher voltages to those bearable by the layer of oxide which isolates it from the internally silicon pastille.

on the gate voltage, with uncertain command

For these reasons there is always a damping resistor in serie al gate, which it is often different for the ignition command and the shutdown. this should be chosen with a compromise between resistance losses on the same (R bassa) and damping necessary to reduce the switching losses (High R), to value through datasheet. With the precautions described now is possible to obtain a fairly rapid descent of the gate voltage, the OFF command: this presents a slight descent "step”, that it would be very pronounced if the command was not decided enough.

Measures for limitation of overvoltages

When a power device turns off quickly, the energy accumulated in the inductance of dispersion is dissipated in the switch static. This causes an overvoltage across the device. The delta transient voltage amplitude is directly proportional to the inductance value of dispersion and to the derivative of current. To limit the overvoltage is therefore necessary to reduce the leakage inductance.

  • Limitation of parasitic inductances of dispersion by appropriate lay-out of the current adductors The leakage inductance due to the current adductors depends on how these are realized; in particular it is noted that it is proportional to their length, which should therefore be limited. Another expedient consists in realizing the adductors by means of overlapping copper sheets and insulated from each other, so as to create a loop that subtends a small area, But what kinds of long power lines, to which corresponds a small inductance.

  • Limitation of overvoltages via low impedance series capacitors places the terminals of the IGBT Referring to the switching off of the IGBT in a bridge inverter, it is seen that the load current tends to go in the free-wheeling diode.

It must also take into account the parasitic inductances, considerable, They have the power adductor connections at the same IGBT, which it is typically a module, as well as distributed over the whole direct current of the inverter power bus. The presence of these inductances generates overvoltages on the IGBT (on VTHIS ) and also to vibrate in resonance with the parasitic capacitances present.

The appropriate insertion of a capacitor prevents the IGBT "sees" the inductances of leakage downstream and upstream of the capacitor itself, so it is advisable to install the capacitors as close as possible to the static

With a filter capacitor electrolytic type, so large capacity, it could serve as a first attenuation of such overvoltage: also, however, it presents a series L. It will therefore; convenient not posit a unique, ma more in parallel, so as to reduce the overall series inductance and make them capable of absorbing the surge voltage on the IGBT through the appropriate electrical charge transits. Even their connection IGBT, however, must be low-inductance: They should therefore be as close as possible, offer a small area between the forward conductor and a return, as well as to make sure that the loop that is formed by the eddy currents is the longest possible. To accomplish this, also taking into account the physical dimensions of the components, the only way is to use copper sheets, that connect directly to the terminals. The two sheets (going + return) They are isolated from each other by a thin layer of polypropylene or mylar, or even fiberglass (1 mm) for high voltages; They are provided appropriate scraps for the connection bolts. To further reduce the parasitic inductances, sometimes he puts another capacitor, reduced capacity to contain the dimensions, directly on the terminals of the IGBT. You are thus obtained the damping of the tension peak.

Adductors current for the IGBT

Limitation of surges through snubber

The snubber is an additional circuit used to reduce the stresses in the power elements. The purpose of the snubber is to improve the transitional, suppressing overcurrent, overvoltage, or improving the derivatives of voltage and current. The snubber dampens the surge in the figure in the turn-off phase, reducing the relative switching losses. II of the snubber capacitor is completely drained net turn-on and turn-off net reloaded: this reduces the dv / dt sull'igbt during the turn-off, allowing one soft switching and reducing losses.

Snubbers RCD

Limitation of surges with clamp

The countermeasure of the electrolytic capacitors typically not enough to protect the IGBT, therefore it becomes necessary another protection, this clamp circuit (fixer). The clamp consists of a capacitor connected in parallel to IGBT, but via a fast diode with regard to the manifold. Such a capacitor is also connected to the positive voltage VCC via a resistor, that it keeps load to this voltage value. If the voltage VTHIS supera VCC, the clamp diode conducts and sets the VTHIS to that of the capacitor, ossia and VCC, the time sufficient to conduct the recirculation diode dell'iverter, which eliminates the problem. After the intervention of the clamp, but before the conduction of the recirculation diode, the voltage tends to rise because; part of the load current goes to end up on the clamp capacitor: must therefore a correct dimensioning of the three security components. The clamp diode, as well as the capacitor, They are actually made up of several components in parallel, mounted on a printed circuit board double-sided, in such a way that it can be also mounted very close to the terminals of the IGBT to be protected.

Clamp circuit and its operation

The problem of protection against overcurrent and desaturation

The IGBTs are virtually improteggibili by fuses: in the time required for a fuse to intervene, the junction of the component will be damaged already irretrievably. And then for a reliable system strength necessary for current limitation. In realizing a protection circuit must take into account some conditions that maximize the efficiency of the protection circuit and that minimize the effects on other circuits. In order to get the security features you want without sacrificing other features, the circuit of protection must, as much as possible, meet the following conditions. First, the protection circuit should turn off the IGBT, before it destroys, every time they come to respect the working conditions of device. Furthermore, must limit the maximum current of a fault and reduce the stresses to the device and the entire system when large currents circulate. II device should be deactivated more quickly if there is no limitation of the maximum fault current.

Outlines of current transducers can be used in power electronics

The current monitoring requires the use of suitable transducers, For example:

– Shunt

It consists of a resistance of value known to be inserted in series with the conductor through which the current passes that you want to measure. II value of the current, DC or AC, It is indirectly calculated according to the Ohm's law by measuring the voltage across the shunt resistor. However, This transducer does not have galvanic isolation between the power circuits and measuring. Furthermore, the level of the shunt voltage is floating to ground. therefore, its use is not easily practicable.

– TA

the oF (amperometric transformer), It is a measuring transformer realized with the necessary precautions to minimize the errors in the current measurement. It ensures the galvanic isolation between the two circuits, but it is not sensitive to the continuous component of the current to be measured.

– Hall effect sensor

The only other solution, It is given that it is also necessary to measure the continuous component of the current, It is to use a Hall-effect sensor. This sensor is a tiny finger of semiconductor material, in which it is slid a constant current I. A 90 degrees with the current, the sensor is hit by a field of magnetic induction B, while on another floor, a 90 degrees with both of the previous, it detects a voltage proportional to, through a constant, induction B is that the current I.

Principle diagram of a current sensor Hall effect

The field B is generated by the current flowing in the component, that it is detected by a magnetic coupling (small toroid), and the extent of this one goes back to the current in the component. The Hall sensor is also sensitive to DC components and has the advantage of not being galvanically connected to the power circuit being measured.

Surge protection and desaturation

– analog and instantaneous current control

The first problem is to detect the current flowing in the IGBT via a transducer. The current signal is used for a closed-loop controller, PID (Proportional Integral Derivative) or hysteresis type (ON/OFF).

– overcurrent limitation by detecting and blocking IGBT

The regulator, in addition to maintaining the current in the load resembling a sinusoid of predetermined effective value, must in any case, if the current exceeds even for an instant, the maximum permissible current, commanding the immediate shutdown of all the IGBTs. This shutdown condition sometimes is meant for a network period, but it often means until further order from the outside, for safety.

– Checking the Status of desaturation of the IGBT (Monitoring of V CE )

Note that the Hall effect detector is not placed in series with the IGBT, but in series with the inverter load. There may thus be cases where the IGBTs are in stark overhead without this current interest load. For example, if one of the IGBT, to a fault, It becomes a permanent short-circuit and what is not detected, power of the other IGBT of the same branch is short-circuits the DC link, with destructive consequences for the entire converter and danger to those who were near. This fault does not involve the load and therefore is not detected by the current sensor. therefore, if the failure occurs upstream of the current transducer to the method illustrated above is no longer effective and it is necessary to control the desaturation of the IGBT. This method consists in monitoring the VTHIS. Indeed, during the short the entire circuit voltage occurs to the IGBT and the V leadersTHIS diverge dal “low on-state voltage”, towards the supply voltage The protection circuit must therefore detect the voltage VTHIS and compare it with a reference value. If the reference is exceeded, the control logic intervenes on the valve control circuit, by requiring shutdown and subsequent power-inhibiting.

It is therefore necessary to integrate the current regulator with the collector-emitter voltage monitoring across each IGBT: an increase over the 2 V of this voltage, when the IGBT is in the ON state, It is a symptom of a probable fault. This symptom can also occur due to faults in the pair of transistors that drive the gate, so it is a protection in most. Put simply a sensor to the heads of the IGBT would not be easy, because it immediately would present a problem of scale:

in state ON, VTHIS worth a few Volt, and here I must evaluate its sudden increases;

in state OFF, VTHIS salt instead to power, or 400, 500, 700 V.

With a full scale of the instrument dimensioned for high power, it is difficult to appreciate with sufficient precision (ie such as to ensure the protection and at the same time avoid spurious tripping) changes in V CE in the ON state. It therefore prepares a more complex circuit, for only measure changes affecting, with a diagram of the type shown in the figure below. The power supply of the control and protection circuits is normally done with dual voltages and point-to-mass central, the order of tens of volts, for example +15/0/-15 V. It is represented the gate control, compound as already described, with the symbol of a generic amplifier, the latter in turn controlled by a control, reported via opto coupler. To explain how to build the signal that arrives at the voltage sensor, We will proceed to next steps, always taking into account that the sensor measures the tension referring to the central supply point. As you will see, this sensor is no more than the input of a voltage comparator. The scheme is in principle; you actually have adaptive changes.

of the circuit diagram for the monitoring of VTHIS

1 step

With resistance R, connected to the positive supply, It is supplied to a voltage sensor that applies:

– +V cc, or a dozen Volt, con IGBT in OFF state

– V about Cesate, ie d.d.t. of IGBT and diode D1 (There was Pochi), con IGBT in ON state

In fact, thanks to the presence of the diode D1, connected in series to the resistance and with the cathode on the collector of the IGBT, during the OFF state of the DC-link voltage can not reach the sensor because this diode is biased against. In the ON state instead D1 conducts and connects the sensor to the IGBT leaders.

2 step

Since it takes a few volt signal, and you want to control such a signal only when the IGBT is commanded to the ON state, introducing the diode D2, with the anode connected to the voltage sensor and with the cathode output opto coupler. As long as it is commanded state of OFF, or the opto coupler output is low (0 logical), D2 conducts and thus the sensor sees only the voltage drop across this diode. When the opto coupler output goes high (1 logical), D2 is cut off and the sensor is connected via D1 to the collector of the IGBT, like before.

3 step

It is also necessary to introduce a small capacitor C, for example by 100 pf, to smooth the voltage spike (modesto!) that occurs in the transition from OFF to ON. In the very short time required to activate the IGBT, the sensor sees interdict D2 and D1 still biased against, then detect the positive voltage supply, now descending. The presence of the condenser eliminates the inconvenience of the peak and the sensor provides a voltage signal always limited to a few volts. By monitoring this signal now, and reacting to increases beyond a certain threshold, one realizes the protection. It compares the voltage signal thus constructed with a reference (obtained it from the control circuits via the setting of a trimmer), by a comparator hysteresis.

From top to bottom:

command at the gate;

voltage without C;

voltage with C

The output of the comparator goes to control the SET input of an SR flip-flop, which in turn serves two functions: the Q output serves as feedback for the control logic, to indicate that a fault has occurred, and to trigger a visual alert (for example, a small LED), that makes it immediately recognizable l IGBT concerned: it; species is useful in many converters with static switches, where it would be complicated to establish dov; the damage; the negated Q output is connected directly to the output of the optocoupler, so as to force her to 0 logic as soon as it detects the fault, without waiting for the intervention of the control logic: thus commands:

the immediate shutdown fault of IGBT. Note that the feedback signal to the control logic must cause a shutdown of the entire converter, because if a static switch remains permanently open, a load side remains floating and there may be problems in subsequent switching periods.

– The problem of the IGBT block in the presence of overcurrent protection circuits In real IGBT we also have that, in case of emergency shutdown due to an overcurrent, the resistor placed in series with the gate gateoff R is higher than the usual value. This is because it intends to limit the overvoltage shutdown causes on the other IGBT of the inverter leg, due to di / dt, overvoltage Puu not be small, since it originates from a higher current than normal. You want to gradually turn off the IGBT suspected fault, because it could happen that we can save it, but it will cause the breakdown of another healthy IGBT for excessive voltage applicatagli

I think I clarified the confusing aspects of the previous article on IGBT

Amilcare Greetings

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