Author Topic: Lower gain feedback transistor outputs more power on HV trans secondary, why?  (Read 181 times)

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Offline gkmaia

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I got a few Lecroy(siglent) WaveAce.

In one of them, the LCD backlight was a bit dim and I suspected the inverter was not supplying enough power to the backlight.

So I replaced Q3 and Q2 for new ones and backlight now is much brighter.

When I replaced them I tested both the old and the new one on my BK curve tracer. These are the results:

- OLD D1802 / Beta: 350 / Display brightness LOW
- NEW D1802 / Beta: 150 / Display brightness HIGH

As Q2/3 drive the oscillation on the primary I am curious to understand why a lower gain transistor on primary/feedback outputs more power on the secondary



 

Offline Zero999

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My guess would be that having a lower beta, it's not driven so far into saturation, therefore will turn off faster, resulting in lower switching losses and higher efficiency, so a brighter light.
 
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