PWM (Pulse Width Modulator) part substitution help

[qsiguy]

I built "Kit67" from my local electronics shop and it works great. I would like to change the output mosfet to allow me to run much higher current. The kit comes with one IRF530 N-Channel Mosfet which is only good for around 7-10 amps. The chip is rated for 16 amps but with other limitations 7-10amps is a safe bet.

I've researched the IRFZ48N which handles 64 amps. I would like to change out the IRF530 for possibly 1, 2, 3, or more of the IRFZ48N's. First question, are there any major mods that would be required to run several of these chips? The circuit uses a 556 timer to drive the mosfet. Best I've been able to figure is that I might need a different value of resistor from the 556 output to each of the IRFZ48N's. Can someone clarify needed mods. Also, does anyone have any other inexpensive suggestions for a high current mosfet to integrate into this circuit (shown below). I can get the IRFZ48N for $1.75 from Mouser and less from Digikey so it's a nice option. I would like to be able to run over 50 amps with minimal heating with the Mosfets on a large headsink w/ a fan like one for a Pentium II CPU

Thanks for any help.

**broken link removed**

 

[audioguru]

Your schematic doesn't show the supply voltage for the 555. If it is about 12V then an IRFZ48N will replace the IRF530 just fine if you change D1 to a 50A diode. Make sure that the wiring can pass 50A and the heatsink is big enough.

 

[qsiguy]

Supply voltage and output voltage will be tied together and will be at about 13.5VDC. Why would you need to change D1 to 50A? The 1N4004 is only a 1A diode and I believe it's only there as a shunt for voltage spikes. Shouldn't be that much amperage going through it ever. Please explain.

So I wouldn't need to change the resistor value on the output from the 556? It's a 10 ohm now. If I install more than one IRFZ48N do I just need to install a 10 ohm resistor on gate of all the mosfets? Any idea how many the 556 would be capable of handling? At some point I'm sure I'd need to power a transistor with the 556 output and power the IRFZ48N's with the transistor.

Thanks.

 

[qsiguy]

Here is a complete discription of this PWM kit.

https://www.electro-tech-online.com/custompdfs/2006/10/ck1400.pdf

 

[audioguru]

The diode passes the same amount of current as the Mosfet each time the Mosfet turns off.

The input current of a Mosfet is zero. But it has a high capacitance that takes a high pulse of current to quickly charge and discharge it.

Paralleled Mosfets each need their own 10 ohm resistor in series with the gate to prevent them from oscillating at a very high frequency.

If too many Mosfets are driven by the 556 then their high input capacitance will overload the 556 and slow down the Mosfets' switching, producing smoke.

A single booster transistor can charge, or discharge the high capacitance but not both. You need 2 booster complimentary emitter follower transistors, an NPN to pull high to charge the capacitance and a PNP to pull low to discharge the capacitance.

 

[qsiguy]

If D1 passes the same current as the Mosfet why did they design a 1A diode with the original IRF530? It's a 16A mosfet.

 

[audioguru]

It is a cheap kit and they don't care if the little diode fails. Most people don't use things at their max.
The PWM frequency is very low so the diode won't be conducting for long.

 

[qsiguy]

Thanks for all your help. One last question, in your expert opinion, how many IRFZ48N's can I run in parallel on the output of the 556 without any additional modifications. 2,3,4, or more? Or should I stick with one? Even if I am going to just run 30-50 amps through it I'd rather have multiple mosfets to keep the heat at a minimum and be able to handle higher amperages at times if necessary.

 

[audioguru]

Use two 50A Mosfets, each with its own 10 ohm resistor in series with its gate.

 

[mira]

Hi all. I am mongolia.
my kurs job name is DC motor drive. than all help me ???

 

[qsiguy]

Sorry Mira, the translation isn't too good. Not sure what your question is. The circuit/kit shown in the schematic is a DC motor controller. I purchased it in a kit form at a local electronics shop. Hard to beat for the price and I will just make some modifications to it to provide greater power handling.

 

[qsiguy]

I'm reviving this thread because I had another question on this circuit. This PWM'er modulates the ground using an NPN output transistor. I would like to modulate the positive side. What modifications would I need to do to swap out the NPN (IRF530) with a PNP so I could modulate the positive lead instead of the negative?

Thanks for any help.

**broken link removed**

 

[qsiguy]

Anyone have any ideas (dumb question, I'm sure you guys do)