Good way to trigger a very large power transistor? - Page 1
Forum section: electronics / projects

author: jdraughn
author_rank: Regular Contributor
timestamp: June 11, 2015, 11:24:46 am
content: Hi all,I got a few QM300HA-2H power transistors from work. They were spare parts for the UPC which got replaced. I have attached the datasheet. They say they are rated for 300 amps and 1000 volts. I would like to try using them in a couple of different projects. One is a speed control for an older 36v golf cart, and the second for a battery powered welder.I would like to drive them from a pic microcontroller, but from looking at the datasheet, to saturate the transistor requires about 4 amps (4 * hFE of 75 = 300 amps). It kind of seems like I need to chain 2 or 3 smaller transistors in front of this big one (which actually contains 3 transistors itself). I am also thinking that it would probably be a good idea to optically isolate my PIC microcontroller.I was also curious if it would be feasible to mount more then one of these transistors in parallel but on different heat sinks and monitor their temperature separately with my PIC, and if one of them starts to get a little warmer then the others then the PWM signal I am sending the transistors would change - it would reduce the duty cycle to the transistor that is warming up while maybe increasing the duty cycle to the ones that are cooler.I don't know the exact specs of the motor in my golf cart, but from searching on the net it appears to be about a 300 amp 36v series wound DC motor (I will find out exactly what motor it is when I get home). Right now the golf cart uses a few solenoids to burn off excess current. I was thinking about using 4 of them to create an h-bridge and then using the large power transistors to modulate the current going through them.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 11, 2015, 11:46:24 am
content: Sorry to say, you'll spend more on the drive circuit alone than purchasing new MOSFETs to handle that motor...You'll also do substantially better than the <90% efficiency you'd get (Vce(sat) < 2.5V at 300A).High power bipolar transistors really are an antiquated industrial method, and not much else. They're almost as slow as SCRs (though quite a bit easier to use), and nowhere near as fast or efficient as IGBTs or MOSFETs. The slow speed and high voltage drop (and voltage rating) mean this device is pretty much only well-suited to motor drive applications -- on the plus side, a sextet of them would handle enough motor to drive a truck, no problem. Don't suppose you happen to have another 19 golf carts to play with, do you?Tim

author: macboy
author_rank: Super Contributor
timestamp: June 11, 2015, 12:29:57 pm
content: These would also be good for a high current and/or high voltage dummy load.

author: Boscoe
author_rank: Frequent Contributor
timestamp: June 11, 2015, 04:49:24 pm
content: Just because the transistor is rated at 300A doesn't mean you will or can use at that. It depends whether your load can draw that current from your voltage rail. It could be a fun project. Design yourself a half bridge driver and attach two together.

author: edpalmer42
author_rank: Super Contributor
timestamp: June 11, 2015, 05:39:54 pm
content: In the datasheet it says that Vce (sat) @ 300A is 2.5V. That's 750 Watts! That's going to be one very large heatsink.Come to think of it, the fan blowing on the heatsink might give you more thrust than the motor.I know! Make an airboat, but with wheels!Ed

author: Zero999
author_rank: Super Contributor
timestamp: June 11, 2015, 07:02:13 pm
content: Those transistors are no good at such low voltages.They're only good for hundreds of volts. The speed can be improved by combining them with a MOSFET to form a cascode.

author: dom0
author_rank: Super Contributor
timestamp: June 11, 2015, 07:24:38 pm
content: Quote from: edpalmer42 on June 11, 2015, 05:39:54 pmYeah, well, that's why it comes in a case with .06 K/W Rth-j-c. Actually, 2.5 V isn't that bad, given it's a triple darlington this implies that the emitter resistance is <1.5 m?.

author: jlmoon
author_rank: Unknown
timestamp: June 11, 2015, 07:39:57 pm
content: IGBT? Use higher voltage , invert while stepping down then pwm 3 phase motor

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 11, 2015, 11:53:55 pm
content: Quote from: dom0 on June 11, 2015, 07:24:38 pm2.5V less ~1V Vbe * two transistors in series leaves about 0.5V (and no more than 1.3V) for actual Vce(sat), which will be limited by the outermost transistor's saturation.It's too bad they don't rate it for more than 10A peak base current; it's probably not painfully slow if you yank the main transistor base down with a generous current (20-100A?). But if you're going to build a driver to do that, you should've sprung for the GTO in the first place...Also, don't be tempted by linear amplifiers either. Not only will you have a hard time finding speakers capable of handling 300V peak,that thing will probably grenade* if you put it in the linear range very far. That is, it probably has significant 2nd breakdown limits.*When you have 600V DC available, with enough electrolytic capacitors behind it to filter line frequency (or supplied by a 3-phase rectifier), electronics don't "release the magic smoke". They explode. That plastic shell turns to pieces. Needless to say, eye protection and a safe distance (and maybe some shields) are recommended for playing with these things. Assuming you have the 600VDC available, of course; that's very much an industrial kind of thing. But a stack of 19 golf cart batteries would be about as destructive, with the advantage of not simply burning until the breaker pops, but delivering cold cranking amps for minutes at a time.Tim

author: moffy
author_rank: Super Contributor
timestamp: June 12, 2015, 12:06:36 am
content: A TIP120 NPN darlington with a TIP125 PNP darlington would drive the base. Use a negative voltage for the PNP of about -3v to -5v, to pull the current out of the base for turn off.

author: jdraughn
author_rank: Regular Contributor
timestamp: June 12, 2015, 07:47:04 am
content: Thanks everyone for your comments and suggestions, I will take them all into account while I figure out a use for these.I did also get 16 used 12v SLA's a year earlier (about 75Ah), some are bulging. I topped them all off with distilled water and they all seem to work pretty dang good for being free. I am using 6 of them in a series/parallel connection as a stick welder. 3 of them in my golf cart, 2 of them in a electric wheel chair that I removed the seat from and is just used to move heavy stuff around. I still have a few batteries left over which I plan on connecting to solar panels and using to power a solar hot water pump and as extra electricity for my house.