4QD controllers Service section and technical information
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Replacement MOSFETs

MOSFET technology gradually improves with time. The MOSFETs we are using in 2005 have a typical R_ds(on) of around 7 milliohms. At the beginning on 2004, the most economical MOSFETs has an R_ds(on) around 12 milliohms.

These MOSFET changes mean that current limit values may have to change to suit, so replacing MOSFETs in the field is not to be recommended. However, for those sufficiently technical this page should give adequate information.

Operating voltages

So clearly the overvoltage limit (which is there to limit the maximum voltage the controller can regenerate under fault conditions, so which must be less than the MOSFET's maximum operating voltage) must be enough to allow the batteries to overcharge. On the low voltage controller, this voltage is set to 47v, and the minimum spec on the MOSFETs is 50. For 48v operation, overvoltage is 68v and the minimum spec on the MOSFETs is 70v.

MOSFET types

This list details the voltages and Rds_(on) of some of the MOSFETs currently and recently used in most 4QD controllers.

1. Colours and * indicate types most commonly in use (September 2012).
2. IR do not quote a typical, but we find this to be around 10 milliohms
3. APEC = Advanced Power Electronics Corp. Of Taiwan. We find these MOSFETs to be far superior to other devices we have used. No value is quoted for 'Rds Typical'.

You will notice that the typical Rds_(on) falls into two groups:

• around 10 Milliohms
• around 6 Milliohms.

You will also note that Vds varies significantly. 55 and 60v devices are used on controllers operating at 36v or lower whilst the 48v controllers require 75v MOSFETs (or higher)

The current limit circuitry on all controllers manufactured by 4QD is somewhat self-adjusting. However, since it uses the MOSFET's own Rds_(on) to limit the current, a value will need changing is you replace a 10 Milliohm MOSFET by a 6 Milliohm.

DIY MOSFET replacement is not therefore recommended and is only for the technically proficient: 4QD can accept no responsibility for the results!

Be aware also that MOSFETs do not usually fail alone. Gate resistors, clamp zeners and driver components also fail as a consequence. See MOSFET failures and MOSFET testing

The drive circuitry used in the NCC and VTX is typical of that used in our other controllers so see NCC series service manual.

MOSFET availability

4QD do not sell spare parts such as MOSFETs but we assume that anyone with sufficient knowledge to repair a controller should have access to suitable MOSFETs - which are freely available from suppliers like Farnell and others.

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