Replace Schottky with Mosfet

Thread Starter

Vindhyachal Takniki

Joined Nov 3, 2014
1. Can I connect 4 diodes in parallel. Max total current is 10A & volatge is 35V I am using PDS1040.

2. However I was reading on internet, then diode Vf decreases as its get more heat up & this may cause current hogging. One solution suggested that add series resistor for each.
3. Can I add 0.010 resistor in front of each diode?
So that max dissipation across in worst case is 10*10*0.010 = 1W.

4. Will 10 milliohm is ok for this application? ( )

Ckt is attached.



Joined Aug 1, 2013
Adding series ballast resistors works only if each resistor is connected to one diode only.
R1 to D1_1 only
R1 to D1_2 only

In other words, separate the anode connections among the diodes.


Thread Starter

Vindhyachal Takniki

Joined Nov 3, 2014
I have made the PCB. Will test it.
But I want to know whether 0.010 ohm resistor is ok to balance the current in case of mismatch

Edit: or should I use higher value resistor.

Roderick Young

Joined Feb 22, 2015
I have made the PCB. Will test it.
But I want to know whether 0.010 ohm resistor is ok to balance the current in case of mismatch

Edit: or should I use higher value resistor.
Since you've already made the PCB, might as well try it. I noticed that you used surface mount diodes. Hopefully your PCB layout provides enough area to carry heat away from those diodes. Best case, each will be dissipating 1.25 watts, and worst case (will never happen) is theoretically 5 watts for one of them. Actually, unless the diodes burn out or the PCB degrades, heat is your friend, as heat means lower voltage drop and lower power dissipation. The PCB is already made, but if you were starting out, I'd say try putting all 4 diodes on the same thermal pad, to share heat.

I would have said don't bother using resistors at all, if you're using 4 of the same type diode. It could be worth trying your circuit out with little pieces of wire instead of the resistors, to start. Again, I see surface mount components, so if you expect the resistors to carry 1/4 watt each, have a wide trace going to them, or even a small copper area on the PCB, to dissipate the heat.


Joined Jun 17, 2014

It is true that a series connected diode does not protect against reverse polarity, but it does protect against reverse current. These are different scenarios as i will try to explain to make this more clear.

Say you have two batteries, one 10v and one 12v. If you connect them to a load by connecting them together the 12v one will try to charge the 10v one and possibly with high current, too high. If you connect a diode anode to the 10v batt and cathode to the 12v batt, the 12v batt will no longer be able to charge the 10v batt so all is well. If the 12v batt goes below 10v (minus the diode drop) the 10v batt will take over the load. At this point the 10v batt will try to charge the 12v batt too, but let's not worry about that. If we did worry about that we'd need another diode, that's all.

But now that we have the diode in series with the 10v batt, say we accidentally connect the 12v batt backwwards, now we have the negative terminal of the 12v batt connected to the cathode of the diode and the positive terminal of the 10v batt connected to the anode (as before) and this means the diode conducts, and conducts heavily. This means something blows out, maybe the diode, maybe something else too.

Here's the circuit the way it normally works:
GND o---- -10+ ----|>|---- +12- ------o GND

and here is with the 12v batt reversed:
GND o---- -10+ ----|>|---- -12+ ------o GND

See now the 10v batt and 12v batt are in series and so conduct a heavy current.

So the protection diode works to prevent reverse current, but no reverse polarity.

The way to protect against reverse polarity (if you really need that though) is to use a very high current diode and a fuse, so that if say the 12v batt is connected in reverse the diode conducts heavy and blows the fuse. The fuse goes in series with the 12v battery.

I had to do this on one of my Mastech power supplies. They are protected against over current, and short circuit, but had only minimal protection against reverse polarity of an external source like a battery. Thus now if you used it to charge a battery and the battery was reverse connected by accident, the fuse blows.

Thread Starter

Vindhyachal Takniki

Joined Nov 3, 2014
Diode in this case is mentioned for reverse current to panel known as dark current. Earlier I thought it also can reverse polarity.
I have made PCB in which there is no series resistor but parallel diode but large this PCB copper area for heat sink. I will test it as it is & see the result.

But in any case how to select which resistor value is suitable. I have attached the pic of If Vs Vf for PDS1040.
In my application maxx current is 10A total with 35V max.
Pic shows max Vf mentioned in red line. So how to calculate most optimal resistor value.