GB2426395A

GB2426395A - Gate driver circuit with under-voltage lockout

Info

Publication number: GB2426395A
Application number: GB0510209A
Authority: GB
Inventors: Kenneth Thompson
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2005-05-19
Filing date: 2005-05-19
Publication date: 2006-11-22
Anticipated expiration: 2025-05-19
Also published as: GB0510209D0; GB2426395B

Abstract

A circuit requiring under-voltage lockout incorporating a gate driver used to drive a component, wherein said circuit includes a comparator to take a measure of the output of said gate driver to determine the under-voltage lockout level; comparator determines the amount of under-voltage lockout by applying an effective hysteresis to the gate driver. The under-voltage lockout is effected by an enable/disable signal from the comparator to the gate driver. The comparator comprises a potential divider comprising two resistances, one end of the divider connected to the gate driver power supply and the other end to an output of the gate driver and the junction connected to an able/disable switch located between a demand signal and the input to the gate driver.

Description

Gate driver This invention relates to gate drivers and has particular but

not exclusive application to MOSF,TETS and IGBT (insulated gate bipolar transistors).

Such devices are generally used as switches. When in operation, if for example, a nominal 15V is applied to the gate, the device is switched to the "ON" state and if OV is applied the device is switched to an "OFF" state. Ideally one does not want to apply intermediate voltages as this reduces effectiveness and potentially damages the transistor due to big voltage and current drops and various anomalies. This is a known problem which it is necessary to reduce. Gate drivers are used to apply the gate voltage and are located between a power supply and a component such as a MOSFET. Furthermore when the power supply to the gate driver is switched on a period of time is required for the output of the gate driver to get up the requisite 15V. Again during this time the low levels of voltage available from the supply through the gate driver can drive the transistor in it's linear characteristic, leading to destruction of the transistor.

Under-voltage lockout is a known method to prevent this problem.

Generally speaking, with such a system, when the power supply voltage is at a lower level than required, the gate driver is disabled so it does not switch on. The problem with this is that as the requisite level is reached the gate driver is continuously enabled and then disabled. To prevent this problem it is known to apply hysteresis to the power supply output by using a comparator with positive feedback. With discrete gate drivers this involves a lot of extra components increasing cost and PCB area for the gate drive.

The invention will now be described with reference to the following figures of which: Figures 1 a) and b) compare and contrast the prior art to the current invention. In both cases a power supply supplies a gate driver. The gated driver has an input designated "demand" which is a signal to switch on or off a device, in this case a MOSFET. In the prior art a hysteresis comparator, (which in the example is powered by a common power supply) is used to apply hysteresis to the system to stop it switching undesirably back and forth especially at undesirable intermediate states as described above. The hysteresis comparator is indicated to have a 12V nominal output and a lower threshold of liv. Thus it steadies the system with an effective variability of 1V. As a result either an enable disable signal is input to the gate driver.

Figure 1 b) shows a schematic representation of a simple embodiment of the invention with similarly numbered components. The new comparator however includes an input signal fed into it from the output of the gate driver itself. In this way the comparator can be set up such that if the voltage of the output driver is at the requisite level, the comparator will determine an appropriate the under-voltage lockout level (e.g. say iv so that the under voltage level is effectively 11 V) and use that to apply an enable or disable signal to the gate driver.

Figure 2 shows a detailed circuit diagram of a simple embodiment of the invention which illustrates the simplicity of the comparator.

A power supply (not shown) supplies both the gate driver 1 and a demand switch 1. The shown is effectively the demand (on or off) signal. The gate driver 2 is shown generally by the broken line. It may or may not include the added resistor configured as shown depending on the set-up.

The comparator under-voltage lockout system is shown by the reference numeral 3. It comprises a Zener diode and a pair of resistors arranged as a potential divider circuit. A transistor 4 implements the enable/disable switching function. The Zener diode determines the under voltage lockout level and the resistors determine the amount of hysteresis. The essential feature is the resistor which is connected to a point giving a measure of the output of the gate driver, such as point "A" which is the output of the gate driver. Of course the skilled person would be aware that what is important is a measure of the gate driver output voltage. Thus the resistor could be connected at any suitable point such as "B" "C" "D". Thus the term "output of the gate driver" includes any such suitable point.

The important feature overall is that some measure of the output of the gate driver provides feedback to determine an under-voltage lockout which is then applied to the input of the gate driver.

Figure 3 shows another example where multiple gate drivers are run off one disabling transistor (Vi 100) with positive feedback resistors Ri 116 and R1i17.

Claims

Claims 1. A circuit requiring under-voltage lockout incorporating a gate

driver used to drive a component, wherein said circuit includes a comparator to take a measure of the output of said gate driver to determine the under- voltage lockout level.
2. A circuit as claimed in claim 1 wherein said comparator determines the amount of under-voltage lockout by applying an effective hysteresis to the gate driver.
3. A circuit as claimed in claim 1 wherein said under-voltage lockout is effected by an enable/disable signal from the comparator to the gate driver.
4. A circuit as claimed in claim wherein said comparator comprises a potential divider comprising two resistances, one end of the divider connected to the gate driver power supply and the other end to an output of the gate driver and the junction connected to an able/disable switch located between a demand signal and the input to the gate driver.
5. A circuit as claimed in claim where the amount of under-voltage lockout threshold (hysteresis) is determined by the ratio of the two resistors.
6. A circuit as claimed in claims 4 or 5 including a Zener diode connected to the junction of the resistors.
7. A circuit as claimed in claim 6 where the under voltage lockout level is generally determined by the Zener diode.
8. A circuit as claimed in any preceding claim including a voltage reference component connected between said switch and the junction of the two resistors.