Low cost mosfet replacement for diode in boost application

Thread Starter

Vindhyachal Takniki

Joined Nov 3, 2014
562
1. have a boost circuit where:
a) Vin: 8.1V to 12.3V
b) Vout = 12.3V/12.5Amps (If 12V/12.5A possible then better, but my ckt gives 12.3V, otherwise need to switch to sepic, which is costly and 12.3V is acceptable so took this low cost part)

2. Here is circuit which I will use. Want to replace D2 diode with mosfet since at 12.5Amps even a Schottky will produce lots of heat.
1599283400651.png


3. I searched for replacement of diode with p channel mosfet like here: https://electronics.stackexchange.c...e-from-a-p-channel-mosfet-and-pnp-transistors

4. Also looked for N-channel mosfet replacement, found some high side driver like these: https://www.microchip.com/paramchartsearch/chart.aspx?branchID=90102

5. But one doubt is mosfet are bidirectional paths, while diode is single side. So will it create issue in circuit?

6. Any other better way to replace with diode here. Need to change in this circuit only. I know there are sync boost converters also, with mosfets, but expenive or if some low cost alternative available?
 

MisterBill2

Joined Jan 23, 2018
7,234
The reverse diode in a mosfet will indeed cause a problem. If you put 2 of those diodes in parallel the drop will be a bit less because of the lower current so less heat and better efficiency. Those diodes are usually mounted on a heat sink in most commercial switcher supplies, which tells me that it is the most cost effective approach.
To get exactly the voltage that you want you can adjust the value of R6, presently 4K7 ohms, a bit. slightly higher resistance gives slightly lower voltage.
 

Papabravo

Joined Feb 24, 2006
14,655
The diode in this converter is a Schottky diode which uses a metal-silicon junction for a couple of very particular reasons. Although it is possible to use other devices in a"diode" configuration, they are unlikely to posses the specific characteristics of the Schottky diode. The most likely effect will be increased ohmic losses, loss of efficiency, and possible destruction of the device. I believe that your efforts in this direction are ill advised.
 

MisterBill2

Joined Jan 23, 2018
7,234
The diode in this converter is a Schottky diode which uses a metal-silicon junction for a couple of very particular reasons. Although it is possible to use other devices in a"diode" configuration, they are unlikely to posses the specific characteristics of the Schottky diode. The most likely effect will be increased ohmic losses, loss of efficiency, and possible destruction of the device. I believe that your efforts in this direction are ill advised.
That is why I suggested two of THAT TYPE of diode in parallel. The Schottkey doides have a much lower forward voltage drop, and also much faster switching characteristics, and both of those properties are very important in switchmode power circuits. Also, the circuit layout is important to the circuit operation. That is why I buy switchers but design and build my own linear supplies.
 

Papabravo

Joined Feb 24, 2006
14,655
That is why I suggested two of THAT TYPE of diode in parallel. The Schottkey doides have a much lower forward voltage drop, and also much faster switching characteristics, and both of those properties are very important in switchmode power circuits. Also, the circuit layout is important to the circuit operation. That is why I buy switchers but design and build my own linear supplies.
I kinda have an aversion to "fixing my house" with clarinet reeds and scotch tape. I understand that the repair parts that you need or want are not always available or affordable, but I think you have to draw a line somewhere and I'm with you on SMPS. I've spent a fair amount of time on them and I still don't think I have a very good sense of all their subtleties, especially stability.

https://www.amazon.com/Steve-Allens-Bop-Fables-Allen/dp/B000GVFWS8
 

MisterBill2

Joined Jan 23, 2018
7,234
There are also published circuits for providing a small boost to a battery voltage as it drops. They use an inverter DC supply in series with the battery. That makes them much more efficient. That function is also available commercially, in a reasonable package.
A quite capable project was published in the "QST" magazine a while back, I don't recall the exact date. That article included details of how to wind the transformer needed for the project. And an inverter, even one with a regulation capability included, is far less demanding than a switching mode power supply.
 
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