In this post we discuss the method of correctly replacing a BJT with a MOSFET, without affecting the final outcome of the circuit.
Introduction
Until MOSFETs arrived in the field of electronics, transistors or BJTs to be precise ruled the power switching circuits and applications.
Though even Bipolar Junction Transistors (BJTs) can not be ignored due to there immense flexibility and low cost, MOSFETs also have certainly become hugely popular as far as switching heavy loads is concerned and due to the high efficiency associated with these components.
However even though these two counterparts may look similar with their functions and style, these two components are completely different with their characteristics and configurations.
Difference Between BJT and MOSFET
The main difference between a BJT and a MOSFET is that, a BJT operation depends on current and needs to be proportionately increased with the load, whereas a mosfet depends on voltage.
But here the MOSFET gets an edge over a BJT, because voltage can be easily manipulated and achieved to the required degrees without much trouble, in contrast increasing current means greater power that's to be delivered, which results in bad efficiency, bulkier configurations etc.
Another big advantage of a MOSFET against the BJT is it's high input resistance, which makes it possible to be integrated with any logic IC directly, no matter how big the load may be that's being switched by the device. This advantage also allows us to connect many MOSFETs in parallel even with very low current inputs (in mA).
MOSFETs are basically of two types, viz. enhancement mode type and depletion mode type. Enhancement type is more frequently used and is the prevalent one.
The N-type MOSFETs can be turned ON or activated by applying a specified positive voltage at their gates while P-type MOSFETs will require just the opposite that is a negative voltage to get turn ON.
BJT Base Resistor vs MOSFET Gate Resistor
As explained above, a the base switching of a BJT is current dependent. Meaning its base current needs to be increased proportionately with increase in its collector load current.
This implies that the base resistor in a BJT plays an important role and must be correctly calculated to ensure that the load is optimally switched ON.
However, the base voltage for a BJT does not matter much, as it can be as low as 0.6 to 1 volts for a satisfactory switching of the connected load.
With MOSFETs it's just the opposite, you can switch them ON with any voltage between 3 V and 15 V, with current as low as 1 to 5 mA.
Hence, a base resistor may be crucial for a BJT but a resistor for the gate of the MOSFET may be immaterial. That said, a low value gate resistor must be included, just to safeguard the device from sudden voltage spikes and transients.
Since voltages above 5 V or up to 12 V are easily available from most digital and analogue ICs, a MOSFET gate can be quickly interfaced with any such signal source, irrespective of the load current.
How to Replace a Transistor (BJT) with a MOSFET
In general we can easily replace a BJT with a MOSFET, provided we take care of the relevant polarities.
For an NPN BJT, we may replace the BJT with a correctly specified MOSFET in the following manner:
- Remove the base resistor from the circuit because we don't typically need it anymore with a MOSFET.
- Connect the gate of the N-MOSFET directly to the activation voltage source.
- Keep the positive supply connected to one of the load terminals, and connect the other terminal of the load to the drain of the MOSFET.
- Lastly, connect the source of the MOSFET to ground.......DONE, you have replaced the BJT with a mosfet within minutes.
The procedure will remain as above even for a PNP BJT to be replaced with a P-channel MOSFET, you will need to just reverse the relevant supply polarities.
Compatible Pinout Replacement Diagram for PNP BJT with P-Channel MOSFET
any replacement for D1 292 transistor. that might be similer to it
You can Google 100V 1 amp NPN transistor, you may find many good options.
Looooongest time sir.
I have been searching for your contacts or your site. I am happy I got you today. You helped me to understand a lot in the past
Thank you Ik, I am always happy to help….. All the best to you.
Hi SwagatamI sent you a msg in facebook asking for help with replacing mrf486’s and mrf422’s i sent it there because i attached a schematic if you could please help thank you
Thank you John,
According to me you cannot replace MRF486 with a MOSFET. You can try searching for 30 MHz 20 Amp Rf NPN power transistor, and find a suitable equivalent for the replacement.
thanks for the reply , yes i been searching and found nothing even close to mrf486’s
i’m thinking they were only made for this one radio only
the next version of this radio that came out had mosfets in it
i compared both schematics to these radios and the pa unit circuit look pretty close to the same design
the thing is i don’t know that much as to how to make the proper modifications to make it work , i never tried it before
Yes, I agree, I too tried searching for “30 MHz power transistor” but could not find anything useful. Unfortunately in RF circuits a BJT cannot be easily replaced with a mosfet.
maybe what i’m looking for is LDMOS not MOSFET’S ?
I think the basic replacement principle will be the same, just as any other mosfet, since LDMOS is also a type of MOSFET.
What ordinary transistor do I replace for Darlington 2SD1298 or D1298?
You can use BU406D and MJE13003 in Darlington pair.
Or you can search for any 400V 10 amp transistor and 400V 1 amp transistor and combine them to make a Darlington pair.
Hi sir how can i connect a universal power module to my smps 30amp cause i cannot get 13009 transistors on my local market
AS far as I know universal power module has a power mosfet or transistors with 3 wires corresponding to the 3 leads of the power device. You can carefully check these wire according to the given user manual and then connect these wires across the respective tracks of the SMPS PCB.
However I would strongly recommend replacing with a discrete transistor instead of a power module.
Hi,
Can you please review the MOSFET symbols given and the type of MOS labeled?
Irfp448n
Good day, i would like to know if its possible to change the 13009 transistors in a smps (cheap 12v 30amp) to N-Channel mosfets, the transistors are driven by a gate transformer. If its possible can you give me a example schematic of how to change it if possible, here is a schematic of the smps.
I would greatly appreciate any help you can give me.
Regards
Darius
Hi, according to me a an 13009 BJT cannot be replaced with a MOSFET in an SMPS circuit.
Thank you for the fast reply, is there no way to rewind the gate drive transformer if I’m correct the transformer is wind in a 4:1 ratio and has a feedback winding aswell, the primary winding is 10v peak to peak and secondary is 2.2v peak to peak, so if you rewind it 1:1 will it work?
Thanks
Regards
Darius
Without seeing the full schematic it can be difficult for me to provide a useful suggestion.
Good day eant to add, primary has 33turns then a center tap with another 33turns after.
Then the secondary has two separate 9turns without a enter tap and then a feed back with 2 turns
The primary and secondary windings has a copper thickness of 0.4mm and the feedback is 0.65mm.
Regards
Darius
Hi, How is this transformer wired, which IC is used? I will need more information regarding your question to figure out the issue?
Hello, the IC is a TL494 and its a half-bridge topology, i see on my first post that i forgot to add the link to the schematic, i’ll add the link in the end.
(imajeenyus.com/electronics/20151028_smps_variable_voltage/index.shtml) or type in on google s-400-12 supply schematic and check on the images.
I hope this help, it has the full schematic and how the gate drive transformer is winded and the main transformer aswell.
Regards and thanks again
Darius
Thanks for the schematic, This is an SMPS circuit where the transformers are the most critical elements of the circuit. You can rewind it but the winding details must be exactly similar to the original data….you cannot change the data with your own specifications.
Hello with respect sir
I have long range cordless phone senao-258 star handy 264MHz 350mW rf power output, transistor is blt50 but i want more power near 2 or 4 watts…
Is it possible to replace ldmos aft504???or please guide me
And another question is i know uhf mosfet kgf-1305, but not found in my area please tell me what are power mos-fet work very similar like kgf-1305
Hello Babak, I could not find any data regarding the mentioned mosfets, so cannot provide any suggestions. You can try other variants such as irf540 etc.