Electronic design of battery charger

David Hull wrote:

Ephemeris wrote:

David Hull wrote:

Robert Krawitz wrote:

Distinctly Average wrote:

Robert Krawitz wrote:

It indicates that the charger doesn't have a diode (essentially a one way valve) to prevent current flowing in reverse through the charger. If nothing else, the lack of that diode means that if the charger is unplugged that it will run the battery down, but it certainly speaks to cutting corners in the design.

It’s not always just a diode. Most modern chargers have the output totally isolated by a mosfet. So the mosfet has to be active for current to flow. The mosfet is enabled when the charger has input power and meets conditions such as thermal limited etc. That is with a good charger. This may be a good charger, but the mosfet has failed shorted so permanently allowing power in both directions.

Showing my age, but it amounts to the same thing in this case -- there should be a valve preventing current flow when it shouldn't (a MOSFET provides more control, preventing forward current flow also if something goes wrong). From other posts, it sounds like this model doesn't have protection on this circuit.

The diode will have a larger forward voltage drop associated with it (about 0.65V) than the MOSFET (which is 0.18V maybe). There are probably some power reduction benifits to using the MOSFET.

Our chargers wouldn't normally perform this function with a discrete device but your point about the tradeoffs to providing output isolation are thermal/power. For some chargers we may have industry standards to adhere to which force some design decisions.

Yea, I agree. If you saw the thread below, there seem to be a lot of descrete parts in the Nikon one, including at least one pretty chunky diode.

https://www.dpreview.com/forums/thread/3715286

It looks quite basic but I suppose it's only expecting to see one type of battery and the current is low.