Hi all!
As you know MOSFET-s are definitely best choice for switching-mode applications. But in most cases they need a gate driver circuit to work properly (of course, if you don't want o make a heater =)). There are lots of driver IC's, but I decided to go another way and test simple discrete driver.
Few days ago I bought some BJT's: SS8050 and SS8550. they are fast (~150-200 MHz), have high collector current (0.7A continuous, 1.5A pulse). Vce is about 20V (most FET's Vgs value).
So I've made two drivers on a breadboard: N-FET low-side and N-FET high-side with bootstrap.
To drive MOSFET(Q1) 8050 and 8550 BJT's are connected as totem-pole emitter follower. I've tested these drivers with 5V signal source, so I've added an inverting level shifter (Q6, R26). If TL494 (for example) is used as control source there is no need in level shift in most case.
Load was 12V 21W auto lamp (~1.8A). MOSFET was without any heatsink. Control signal: square wave generator with PWM on TL7555 timer with output buffer (74HC04). Frequency: 2KHz to 70 KHz.
Low side.
Classic bootstrap driver. Works fine up to 70 KHz (90 KHz not tested). Also at frequency >50 KHz FET is very warm. FET temperature vs. frequency is nearly low side results.
Summary.
These discrete drivers work fine at ~40-60 KHz =)
As you know MOSFET-s are definitely best choice for switching-mode applications. But in most cases they need a gate driver circuit to work properly (of course, if you don't want o make a heater =)). There are lots of driver IC's, but I decided to go another way and test simple discrete driver.
Few days ago I bought some BJT's: SS8050 and SS8550. they are fast (~150-200 MHz), have high collector current (0.7A continuous, 1.5A pulse). Vce is about 20V (most FET's Vgs value).
So I've made two drivers on a breadboard: N-FET low-side and N-FET high-side with bootstrap.
To drive MOSFET(Q1) 8050 and 8550 BJT's are connected as totem-pole emitter follower. I've tested these drivers with 5V signal source, so I've added an inverting level shifter (Q6, R26). If TL494 (for example) is used as control source there is no need in level shift in most case.
Load was 12V 21W auto lamp (~1.8A). MOSFET was without any heatsink. Control signal: square wave generator with PWM on TL7555 timer with output buffer (74HC04). Frequency: 2KHz to 70 KHz.
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| Low side driver |
Load is connected to Vcc and Q1 drain. PWM - 40-90%
Just for test. 90 KHz - hot, heatsink requred. I think, that for frequency >65 KHz special driver IC is a better choice.
High side.
 |
| High side discrete driver |
Summary.
These discrete drivers work fine at ~40-60 KHz =)
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