Abstract: Through damages mechanism’s analysis to IGBT, according to its damage’s reason, takes the corresponding measures to carry on the protection to it, guarantees its security reliable work.
Key word: IGBT; MOSFET; Actuation; Crosses the pressure; Surge; Cushion; Overflow; Overheated; Protection
Introduction
Insulation grid double pole transistor IGBT is one kind of component which bonds by MOSFET and the double pole transistor becomes, its input extremely MOSFET, output extremely PNP transistor, therefore, may regard as it is MOS the input Darington tube. It fused these two kind of component’s merit, both has the MOSFET component to actuate simple and the fast merit, and has the double pole component capacity big merit, thus, obtained more and more widespread application in the modern electric power electronic technology.
In high efficiency switching power supply installment, IGBT, because its control driving circuit is simple, the operating frequency is high, the capacity major characteristic, has substituted for the thyristor or GTO gradually. But in switching power supply installment, because it works in high frequency with the high voltage, under the big electric current condition, causes it easily to damage, moreover, before the power source takes system’s level, because reason and so on electrical network fluctuation, thunder stroke influences causes the stress which it withstands to be bigger, therefore IGBT reliable direct relation power source’s reliability. Therefore, when chooses IGBT besides must do falls the volume consideration, the protection design is also the power source designs when needs the key consideration to IGBT a link.
1 IGBT principle of work
IGBT equivalent circuit as shown in Figure 1.
May know by Figure 1, if adds on the actuation positive voltage between the IGBT electronics grid and the emitter electrode, then the MOSFET breakover, between such PNP transistor’s collecting electrode and the base electrode becomes the low resistivity condition to cause the transistor breakover; If between the IGBT electronics grid and the emitter electrode the voltage is 0V, then the MOSFET closure, shuts off the PNP transistor base current the supplies, causes the transistor closure.
Thus it may be known, the IGBT security is reliable or not mainly decided by the below factor:
- - Between IGBT electronics grid and emitter electrode’s voltage;
- - Between IGBT collecting electrode and emitter electrode’s voltage;
- - winds through the IGBT collecting electrode - emitter electrode’s electric current;
- - IGBT junction temperature.
If between the IGBT electronics grid and emitter electrode’s voltage, namely the slaving voltage is excessively low, then IGBT cannot stabilize works normally, if between excellent electronics grid - emitter electrode’s pressure resistance IGBT possible irremediable defect; Similarly, if adds, in the IGBT collecting electrode and the emitter electrode permission’s voltage surpasses between the collecting electrode - emitter electrode’s pressure resistance, will wind through the IGBT collecting electrode - emitter electrode’s electric current to surpass the collecting electrode - emitter electrode permission the maximum current, the IGBT junction temperature surpasses its junction temperature the permissible value, the IGBT possibly irremediable defect.
2 protective measures
When carries on the circuit design, should aim at affects the IGBT reliable factor, takes the corresponding protective measures with a clear goal.
2.1 IGBT electronics grid’s protection
IGBT electronics grid - emitter electrode slaving voltage VGE guarantee value for ±20V, if adds on between its electronics grid and the emitter electrode surpasses the guarantee value the voltage, will then possibly damage IGBT, therefore, must establish the grid-voltage limiter circuit in the IGBT driving circuit. Moreover, if the IGBT electronics grid and the launch interelectrode opening, but adds on the voltage between its collecting electrode and the emitter electrode, along with collecting electrode electric potential change, because between the electronics grid and the collecting electrode and the emitter electrode parasitic capacity’s existence, causes the grid potential to elevate, the collecting electrode - emitter electrode has the electric current to wind through. If by now the collecting electrode and the launch interelectrode were at time the high-pressured condition, possibly will cause IGBT to give off heat even damages. If the equipment causes the grid return separation in the transportation or the vibration process, in was not realized in the situation adds on the voltage to the main circuit, then IGBT possibly will damage. In order to prevent this kind of situation occurrence, should connects in parallel fashion several dozens kΩ resistances in the IGBT electronics grid and the launch interelectrode, this resistance should approach the electronics grid and the emitter electrode as far as possible. As shown in Figure 2.
Because IGBT is power MOSFET and the PNP bipolar transistor’s complex, specially its electronics grid is the MOS structure, therefore besides the above proper protection, looks like other MOS structure component to be the same, IGBT regarding the static voltage is also very sensitive, when also carries on the erecting welding work to IGBT must pay attention to the following item:
- - before needs to use the hand to contact IGBT, after should first human body’s on static discharge, carries on the operation again, and do not contact the module as far as possible the actuation post part, must contact when must guarantee the static electricity which this time on the human body brings to bleed off completely;
- - when welds the work, to prevent the static electricity possibly to damage IGBT, the welding machine must certainly earth reliably.
2.2 collecting electrodes with launch interelectrode’s overvoltage protection
The overvoltage production mainly has two kind of situations, one kind is exerts the IGBT collecting electrode - to launch interelectrode’s direct-current overtension, another kind for collecting electrode - emitter electrode’s on surge overtension.
2.2.1 direct-current overvoltages
Has directed current the reason which the pressure produces is because before the input alternating current supply or the IGBT, first-level input occurs exceptionally is the result. Solution is when selects IGBT, carries on falls the volume design; Moreover, may when examine this presses has breaked the IGBT input, guarantees IGBT the security.
2.2.2 surge voltage protection
Because in electric circuit distributed inductance existence, the IGBT shutter speed is in addition high, when IGBT shutdown and when reverse resumes the diode backward recovery with it joining-up, will produce very big surge voltage Ldi/dt, threatens IGBT the security.
Usually IGBT surge voltage profile as shown in Figure 3.
In chart: vCE is IGBT? The electrode - launches interelectrode’s voltage waveform;
ic is the IGBT collecting electrode electric current;
Ud is inputs IGBT the DC voltage;
VCESP=Ud Ldic/dt, is the surge voltage peak value.
If VCESP surpasses IGBT collecting electrode - launch interelectrode pressure resistance value VCES, damages IGBT on the possibility. Solution mainly has:
- - when selects IGBT considers the design allowance;
- - when circuit design adjusts IGBT driving circuit’s Rg, causes di/dt to be as far as possible small;
- - approaches as far as possible the electrolysis electric capacity the IGBT installment, reduces the distributed inductance;
- - basis situation addition cushion protection circuit, bypass high frequency surge voltage.
As a result of the cushion protection circuit to the IGBT trouble-free service very vital role, makes in the cushion protection circuit’s type and the characteristic an introduction.
- - C cushion circuit as shown in Figure 4(a), uses the film capacitor, approaches the IGBT installment, its characteristic is the electric circuit is simple, its shortcoming is constitutes the LC acceptor by the distributed inductance and the cushion electric capacity, easy to have the voltage vibration, when IGBT clear the collecting electrode electric current is big.
- - RC cushion circuit as shown in Figure 4(b), its characteristic is suits in cuts the wave electric circuit, but when uses large capacity IGBT, must make the cushion resistance value to increase, clear the collecting electrode electric current is oversized, causes the IGBT function to receive certain limit.
- - RCD cushion circuit as shown in Figure 4(c), compares its characteristic with the RC cushion circuit is, increased the cushion diode, thus causes the cushion resistance to increase, has avoided when the clear the IGBT function is blocked question.
In this cushion circuit the cushion resistance produces the loss is
P=LI2f CUd2f in the formula: In L primarily electric circuit’s distributed inductance;
I is time the IGBT shutdown collecting electrode electric current;
f is the IGBT turn-on frequency;
C is the cushion electric capacity;
Ud is the DC voltage value.
- - electric discharge impediment cushion circuit as shown in Figure 4(d), compares its characteristic with the RCD cushion circuit is, produces the loss is small, suits in the high frequency switch.
The loss which in the cushion resistance produces in this cushion circuit is
P=1/2LI2f 1/2CUf
According to the actual situation selection suitable cushion protection circuit, suppresses the shutdown surge voltage. When carries on the assembly, must reduce the main circuit and cushion circuit’s distributed inductance as far as possible, the wiring is shorter thickly is better.
2.3 collecting electrode electric current overflow protection
To the IGBT overflow protection, mainly has 3 methods.
2.3.1 use the resistance or the current transformer examination overflow carries on the protection
Like Figure 5 (a) and Figure 5 (b) shows, may use the resistance or the current transformer and the IGBT series, the examination winds through the IGBT collecting electrode the electric current. When has the overflow situation occurs, the control implementing agency separates IGBT the input, achieves protects IGBT the goal.
2.3.2 carry on the protection by the IGBT VCE(sat) examination overflow
Like Figure 5 (c) shows, because of VCE(sat)=IcRCE(sat), when Ic increases, VCE(sat) also increases along with it, if the grid bias is the high level, but VCE is high, then this time has the overflow situation occurrence, this time and gate outputs the high level, the overflow signal output, the control implementing agency separates IGBT the input, protects IGBT.
2.3.3 examination load current carries on the protection
This method and Figure 5 (a) examination method basic same, but Figure 5 (a) is a direct method, this is an indirect method, like Figure 5 (d) shows. When the load short-circuits or the load current enlarges, before also possibly causes, the level IGBT collecting electrode electric current to increase, causes IGBT to damage. (Or the IGBT earlier stage electric circuit) examines after the load place exceptionally, the control implementing agency shuts off IGBT the input, achieves the protection the goal.
2.4 overheated protections
The ordinary circumstances obscene IGBT electric current has been big, the turn-on frequency is high, therefore component’s loss is also quite big, if the quantity of heat cannot disperse promptly, causes component’s junction temperature Tj to surpass Tjmax, then IGBT possible to damage.
The IGBT power loss and dynamic moves including the stable state power loss consumes, its dynamic power loss includes the clear power loss and the shutdown power loss. When carries on the hot design, not only need guarantee it when normal work can radiate fully, moreover must guarantee it when has the momentary overload, the IGBT junction temperature does not surpass Tjmax.
Certainly, equipment’s volume and the weight and so on limit as well as the performance-to-price ratio consideration, the cooling system expands unlimitedly not possibly. But is approaching a IGBT place addition temperature relay and so on, examines IGBT the operating temperature. The control implementing agency when occurs exceptionally shuts off IGBT the input, protects its security.
In addition, installs IGBT toward the radiator on is fixed when should pay attention the following item:
- -, because the thermal resistance installs the position along with IGBT the difference and the difference, therefore, if only installs time IGBT on the radiator, should install it in, with the aim of causing the thermal resistance to be smallest; When must install several IGBT, should act according to each IGBT to give off heat the situation to keep the corresponding space;
- when - use belt trace radiator, should be suitable radiator’s trace the IGBT wide direction, reduces radiator’s distortion;
- - radiator’s installment surface smooth finish should ≤10μm, if radiator’s surface is uneven, will increase the radiator and component’s contact thermal resistance greatly, even has the very big tensity between the IGBT tube core and on the shell substrate, damages IGBT the insulating layer;
- - to reduce the contact thermal resistance, should better smudges the heat conduction silicon fat in the radiator and the IGBT module.
3 conclusions
When using IGBT should act according to the actual situation, takes the corresponding protective measures. So long as after the pressure, the overflow, overheated and so on several aspects take the effective protective measures, in the practical application can make the good progress, guaranteed that IGBT works reliably safely.
