biasing a mosfet or heating issue
biasing a mosfet or heating issue
(OP)
Hello,
I am having trouble with the power consumption by a paired mosfet array chip. The should be simple circuit is attached. The mosfet array (half bridge chip – FMP26 by IXYS) is being switched at 1-2MHz and thus, a 12A pin driver (EL7158) is being used to deliverer more current to the larger gate capacitance at the high frequencies (under 500khz the pin driver is not needed). So this is a high current, 10V output square wave of around 1Mhz used to drive each of the mosfet gates.
As I gradually increase the voltage on the P channel source up to 50-60V, the mosfet gradually gets hot, and this is without a load on it.. Now this effect is reduced by lowering the resistor (100 ohm in attached figure) value which is biasing the gate, reduces VGS by creating DC offset on the gate which lifts the pulse waveform. Im not sure what is going on though and how to properly bias this as 100 ohms is very low. Shouldn't I be able to switch it with these VGS or whatelse is the problem
Mosfet datasheet
http:/ /ixdev.ixy s.com/Data Sheet/DS10 0033A(FMP2 6-02P).pdf
circuit diagram
http ://files.e ngineering .com/getfi le.aspx?fo lder=980ce 9ab-c909-4 8ad-99f7-b 8252fb6ee6 7&file =fmp26.bmp
oscilloscope waveform 60V
http ://files.e ngineering .com/getfi le.aspx?fo lder=01982 674-c60f-4 d22-8597-8 f638882e23 1&file =IMG_20110 609_182149 _small.jpg
many thanks
jim
I am having trouble with the power consumption by a paired mosfet array chip. The should be simple circuit is attached. The mosfet array (half bridge chip – FMP26 by IXYS) is being switched at 1-2MHz and thus, a 12A pin driver (EL7158) is being used to deliverer more current to the larger gate capacitance at the high frequencies (under 500khz the pin driver is not needed). So this is a high current, 10V output square wave of around 1Mhz used to drive each of the mosfet gates.
As I gradually increase the voltage on the P channel source up to 50-60V, the mosfet gradually gets hot, and this is without a load on it.. Now this effect is reduced by lowering the resistor (100 ohm in attached figure) value which is biasing the gate, reduces VGS by creating DC offset on the gate which lifts the pulse waveform. Im not sure what is going on though and how to properly bias this as 100 ohms is very low. Shouldn't I be able to switch it with these VGS or whatelse is the problem
Mosfet datasheet
http:/
circuit diagram
http
oscilloscope waveform 60V
http
many thanks
jim





RE: biasing a mosfet or heating issue
The fact that you report heat with no load sounds suspicious. Unless the gate drive itself is causing heat (?).
PS: your scope is from the future...
RE: biasing a mosfet or heating issue
Scott
I really am a good egg, I'm just a little scrambled!
RE: biasing a mosfet or heating issue
To prevent an overlap you may need to try different value gate resistors for the P and the N channnel devices instead of both being 100 ohms (see data sheet). This can help offset the differences in turn-on and turn-off times between the two device types. To reduce spurious oscillations a small resistor in series with each gate e.g. 1 or 2 ohms may help.
RE: biasing a mosfet or heating issue
Try to connect some ceramic capacitors using a low inductance layout technigue.
You have to regard such circuit as Rf-circuit although it's "only" 1 MHz.
RE: biasing a mosfet or heating issue
RE: biasing a mosfet or heating issue
1) From the circuit schematic, it looks to me that the On gate drive is 5 V. The gate turn on threshold is speced at 2.5 to 5 V.
2) Overlap of the FET turn on / turn off delays can cause both FETs to conduct briefly. Some delay must be added between turn on and turn off.
RE: biasing a mosfet or heating issue
Z
RE: biasing a mosfet or heating issue
You also need a resistor in series with each gate too. The data sheet says 3.3ohms for the N-channel and 10ohms for the P-channel.
I too, would expect to see a 5V gate drive, which would actually mean +5V to -5V. The 0.1uF gate caps will charge to some DC value in operation. I would expect this to be 5V but it could easily end up as some other value.
RE: biasing a mosfet or heating issue
I also thought that it's a dead-time as well. It's possible that there's a shoot through. I had this problem on my buck converter, the high side transistor gets extremely hot under no load.
RE: biasing a mosfet or heating issue
RE: biasing a mosfet or heating issue
How do you come to conclusion 1) ?