Transistor desaturation protection using

the driver undervoltage lock out

Alex Van den Bossche1, Angel St. Marinov2, Vencislav C. Valchev3

Abstract

In the gate drive circuits, a lot of choice is available in simple drivers in 8 pin devices. If also a desaturation protection is needed, the choice is greatly reduced. However, almost all gate drivers also have an undervoltage lock-out. The principle is to have a weak supply and to use pull the supply down for some time. It is mainly useful in converters which combine slow and fast switching devices and where the desaturation protection is done at the slow
switching devices. It is intended to be applied in reliable but cost effective distributed generation converters.

1. Introduction

There are many power electronics applications, where there has to be a galvanic separation between the power and control stages. The galvanic separation provides security for the control stage by decreasing the chance of harmful disturbance being transferred from the power stage, as well as increasing the safety for the consumer by separating the high current, high voltage power stage from the low voltage; low current control stage which has to be accessible for the consumer. The galvanic separation between the two stages is done at the driver stagewhich can be considered part of the power stage. A convenient way is to use an optocoupled driver. The optocoupled driver can be a single IC or two separate IC one for driving the transistor and one for optocoupled galvanic separation.

Except the galvanic separation, a must for every high power converter is a fast and well build current protection which can act adequately for overcurrent as well for short circuit. The current protection can be made on the control stage or directly integrated in the power stage.

When protecting directly at the power stage one can achieve a slightly faster and more reliable protection without the need of an extra galvanic separation that will be needed to transfer information for the current on the transistors, to the control stage. One of the easiest ways of sensing current for the means of protecting, especially for IGBTs is by monitoring the desaturation of the transistor which is in a direct relation with the current trough the device, thus creating a desaturation protection.

The purpose of this paper is creating a discrete desaturation protection build on the power stage which interacts with the optocoupled driver in order to initiate protection. Such protection should be build to be competitive in terms of versatility, employment, price and working parameters compared to integrated protections in specialized optocoupled drivers.

2. Concept and principle of operation.

The protection presented in this paper was originally intended to be used in a reliable but cost effective distributed generation converters. It’s main purpose – to be a self restarting protectionfor a slow switching IGBT in a pulse width modulated converter. First, optocoupled drivers with an integrated desaturation protection were considered. Such ICs pack a high variety of functions in a rather small package but are sold at relatively high price and are not always corresponding to the real needs like low losses. Some of them have complicated controls and are intended for µP control. The design presented in this paper on the other hand offers a simple and cost effective discrete solution which can be used with a variety of optocoupled drivers, as long as they have UVLO (common feature for any driver). The purposed protection acts fast and can protect power components as MOSFETS and IGBTs from overcurrent as well as short circuit, therestart time and limiting current can be adjusted by simple change in components values.The schematic of the protection is shown at fig. 1. The driver used in the purposed schematic is HCPL3120. It is a simple eight pin optocoupled gate drive able to source and sink currents of 2A for the purpose of driving MOSFET or IGBT. The protected transistor is a medium power IGBT – IRGP4062D. Those two components were used for testing and as a part of the system that the protection is going to be included in.

1 Alex Van den Bossche, Ghent University, Belgium.

2Angel Marinov, Technical University of Varna, Varna, Bulgaria, E-mail:

3Vencislav C. Valchev, Technical University of Varna, Bulgaria,

They are not crucial for its work and can be changed depending on the application that the protection is needs to be used in. The voltage drop on the protected transistor Q1 is sensed by the high voltage diode D2. The protection is activated when the voltage drop across Q1 exceeds the protection threshold which is set by the gate drive voltage and the voltage divider R2,R4 and R5. At the limit current,the transistor Q2 starts conducting,leading to the discharge of capacitor C2. This results in pulling down the gate of the P-channel MOSFET Q3, thus turning on the transistor and pulling down the supply voltage of the driver +V trough the resistor R1. The reduced supply on the driver causes it to go into undervoltage lockout (UVLO), stopping the gate drive of transistor Q1 and securing it from overcurrent by turning it off. After the gate voltage stops transistor Q2 stops conducting allowing capacitor C2 to start charging from the supply +V via resistors R6. This sets a restart time of the protection. When this time passes and C2 charges to certain level Q3 turns off and the gate drive resumes its normal operation as long as the voltage drop on the protected transistor Q1 stays below the set threshold.

Fig 1. Desaturation protection schematic

The current threshold of the protection can be determined by the values of resistors R4 and R2, for precise adjustment R4 can be replaced by a trimmer. The restart time of the protection is determined by the values of capacitor C2 and resistor R6, where R6 should be chosen relatively high to keep currents low. C2 should be chosen low to have deeper discharges at low current trough Q2. Temperature dependence in the circuit is introduced by the mainly by the protected transistor since its relation between Collector emitter voltage drop and collector current is temperature depended.

There are three extra elements which are used to create a more stable and reliable operation of the protection. CapacitorC3 has value of several hundred Pico farads and it is used to filters disturbances that can be transferred to the base of Q2 and create a false start of the protection. Capacitor C3 is crucial to the start time of the protection, it has to be chosen small enough so not to interfere with the start time of the protection but at the same time big enough so it won’t allow false starts. Diode D3 is used to sink the charge accumulated in the parasitic capacitance of diode D2 on gate drive turn-off. If the diode D3 was not present, the protection would be activated on Q1’s turn-off due to the charge in D2 which wouldbe transferred to the base of Q2.

The waveforms in fig.2a and fig.2billustrate the described principle of operation. Both graphs were built in MATLAB using data collected by measuring several points of the protection with digital oscilloscope – TSP2042. The data for both graphs was obtained for the following test conditions: control frequency: 50Hz; duty ratio: 25%; Working voltage of the protected transistor: 300V; set current of the protection 13A. Low frequency was used so the time needed to restart the protection can be properly observed (since it is 1ms). Low initial duty ratio and set current was used to avoid unintended overload in the transistor or the load. Fig. 2a shows how the protection works over time, if the overcurrent remains, by limiting the average current trough the transistor while Fig 2b is a zoomed version of Fig.2a. Following the traces in Fig.2b, when the current trough the protected transistor reaches the set value of 13A, capacitor C2 discharges to nearly 5V turning transistor Q3 on, which on the other hand pulls down the supply forcing the UVLO of the driver to shut down the gating signals.

a)b)

Fig. 2. Protection Waveforms

3. Test results and comparison between integrated and discrete optocoupled desaturation protections.

The suggested protection has been tested for various transistor currents and voltages. Results show that the protection acts adequately for overcurrents as well as for real live short circuits, leaving the protected transistor unharmed and safely restarting after the set time. The protection can be used for various protection currents, and is stable across a high frequency range. The value of the protection current shows small variations depending on the working voltage of the protected transistor. Fig. 3 shows how the set protection current depends on the working voltage, and Fig.4 shows a set protection current of 13A for three different frequencies.

Fig.3 Protection current VS Working voltage

Fig.4 Protection current VS Working voltage in frequency dependence.

4. Conclusions.

Test results showed that the protection is functional, adequate and quite robust. It was implemented in number of power modules used in different converters, where it proved capability and stability in real working conditions. When build in high numbers the protection showed little tolerances in its main properties.

Compared to desaturation protections integrated in drivers, the suggested discrete protection is price comparative with the optocoupled IC drivers and close to the prices of drivers without galvanic separation. It has low complexity and does not require any additional controls to operate. The suggested protection can be used with multiple number of integrated drivers witch allows flexibility in current capability and losses. The protection time is not restarted at each pulse, so making it immune for failures by erronous high frequency control signals. The size can be reduced if the protection is build in SMD.

5. Acknowledgments.

This paper was developed in the frames of the research project N21 'Improvement of research potential in the area of engineering and IT’ in the frames of the Program ‘Development of human resources’, BG051PO001/07/3.3-02, Ministry of Education, Bulgaria.

6. References

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