TL494 directly driving Mosfets (OLD AMP)

Lenin89

Member
2019-03-03 2:33 pm
Varna
I'm repairing an really old amp (at 25years+ old, maybe even more).
PS section was totally destroyed.
I don't know the original gate resistor values and ps mosfets. TL494 was also dead (originally it was Toshiba TL494).

So i've replaced the TL494, fitted 47 ohm gate resistors and 4 irfz44. Initially there were no proper gate drive. I've noticed there are no any buffer driver transistors.

TL494 drives directly the mosfets through it's E1 and E2 pin 9 & 10. There is only two resistor to ground from respectively pins 9 & 10 and a gate resistor per mosfet.

Originally the resistors to ground were big ones 2W at around 100ohm. With these there were no gate drive and the IC was getting hot.

I've replaced these ones with a 1/4W 200ohm (that's what i've had laying around). This gave a proper gate drive, but the mosfets now are heating way too fast and way too much.

As I'm not an expert, I'm suspecting that the poor little IC TL494 can't handle that much of a load in that setup.

I know it's 100x times better to have a buffer transistors, but reading the datasheet of tl494 and threads here and there over the web, TL494 can drive a light load in the right condition.

If I match the resistors to ground to gate resistors and to the right mosfets
is there any chance I can get this PS working without ! any buffer transistors and modding the schematics ?

Today I will try using resistors to ground ~ 500ohms or 1kohm to see how this goes...
 
Last edited:
I'm repairing an really old amp (at 25years+ old, maybe even more).
PS section was totally destroyed.
I don't know the original gate resistor values and ps mosfets. TL494 was also dead (originally it was Toshiba TL494).

So i've replaced the TL494, fitted 47 ohm gate resistors and 4 irfz44. Initially there were no proper gate drive. I've noticed there are no any buffer driver transistors.

TL494 drives directly the mosfets through it's E1 and E2 pin 9 & 10. There is only two resistor to ground from respectively pins 9 & 10 and a gate resistor per mosfet.

Originally the resistors to ground were big ones 2W at around 100ohm. With these there were no gate drive and the IC was getting hot.

I've replaced these ones with a 1/4W 200ohm (that's what i've had laying around). This gave a proper gate drive, but the mosfets now are heating way too fast and way too much.

As I'm not an expert, I'm suspecting that the poor little IC TL494 can't handle that much of a load in that setup.

I know it's 100x times better to have a buffer transistors, but reading the datasheet of tl494 and threads here and there over the web, TL494 can drive a light load in the right condition.

If I match the resistors to ground to gate resistors and to the right mosfets
is there any chance I can get this PS working without ! any buffer transistors and modding the schematics ?

Today I will try using resistors to ground ~ 500ohms or 1kohm to see how this goes...


Generally, in designs that use a buffer, the resistor to ground has a high value (1/2 / 10k) after the buffer.
Since TL494 is fragile and does not have a great ability to drive mosfets by itself, the 200ohm resistor is a risk, it puts a strain on outputs that are already very weak.
Usually TL494 manages to turn on the mosfets well but does not turn them off correctly, for this reason a buffer (single ended) is used with the help of a diode.
In your case, try using a 1k to ground and 68ohm resistor for the gate if you use IRFZ44N.
If you're still having trouble, consider making a modify to add a buffer stage.
 

Perry Babin

Member
Paid Member
2003-11-20 11:01 am
Louisiana
www.bcae1.com
This sounds like an Alpine. What is it?

Alpine uses a 110 ohm pulldown resistor and it works fine. The 494 can drive high but has very little ability to drive back to ground. That's why they use the low-value pulldown.

You should have had a good drive signal with the 100 ohm pulldowns. Reinstall the 100 ohm resistors and confirm that you have 12v on pins 8 and 11.
 

Lenin89

Member
2019-03-03 2:33 pm
Varna
I will take photos of the amp when I get back home, it's a brand i've never heard before.
With the two 100ohm pull-downs the gate drive wasn't right. It was almost DC. That's why i've removed them initially. Bear in mind that they read right at 100ohm on DMM.

So i've replaced those two with 200ohms and gate drive came back to life as it should be.
With these the gate drive looks OK, but the output mosfets are heating a lot even for 10 seconds. I think there is a lot of overshooting.
 

Lenin89

Member
2019-03-03 2:33 pm
Varna
Picture of the amp + gate drive without mosfets and without resistors to ground .
 

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