I have a Boston Acoustics Soundwave subwoofer that I'm trying to resuscitate. It was blowing fuses and I narrowed the problem down to the main switching power supply which had blown MOSFETs. These are controlled by an IR21531S.
My understanding of these ICs is they work similarly to a 555 timer and provide constant, and opposing-phase frequency to a high and low output, in this case 50KHz.
What I found was that the LO was oscillating normally but the HO was sat at a constant 12v. This apparently was causing the HO-linked FET to fail. OK, it's a blown 21531, these are cheap so I'll replace that. This board has the oscillator on a little daughterboard which makes testing much more straightforward.
However, the replacements are behaving in exactly the same manner. Now I will confess I bought them from AliExpress because I didn't fancy the £12 shipping fee from DigiKey, however given the original part, and at least two of the replacements are exhibiting exactly the same condition I have a hard time believing the fault is with all three ICs.
Am I missing something fundamental here? The HO and LO outputs are direct from the chip, and given one is working I think I can discount the rest of the circuit, but this has me baffled.
Circuit diagram from the service manual attached.
My understanding of these ICs is they work similarly to a 555 timer and provide constant, and opposing-phase frequency to a high and low output, in this case 50KHz.
What I found was that the LO was oscillating normally but the HO was sat at a constant 12v. This apparently was causing the HO-linked FET to fail. OK, it's a blown 21531, these are cheap so I'll replace that. This board has the oscillator on a little daughterboard which makes testing much more straightforward.
However, the replacements are behaving in exactly the same manner. Now I will confess I bought them from AliExpress because I didn't fancy the £12 shipping fee from DigiKey, however given the original part, and at least two of the replacements are exhibiting exactly the same condition I have a hard time believing the fault is with all three ICs.
Am I missing something fundamental here? The HO and LO outputs are direct from the chip, and given one is working I think I can discount the rest of the circuit, but this has me baffled.
Circuit diagram from the service manual attached.
Attached Files
Comment