4QD series controllers - Current limits

Contents, this page

• Earlier Controllers
• Control board layout (issue 13 shown)
• Later Controllers

Earlier Controllers

The 4QD series controllers were originally aimed at golf buggies and similar terrestrial applications where reverse is not used much and reverse current does not need to be as high as forward current. The 4QD series use a full bridge circuit and since the forward hiside is conducting forward current all the time and is not chopped, this is where most heating will occur and if MOSFET current limits were ever pushed to the limits, this is the first place than should fail. So the bridge includes one extra MOSFET in this arm to reduce the dissipation here.

The ratio of forward to reverse current in the standard controllers was set to 70% and the values have never been changed since. A reverse current 75% of forward would be correct for a 3 MOSFET per arm bridge, with 4 MOSFETs in the forward hiside. This is the compliment of the 4QD-150 which has been the most common of the family, but the -200 uses 4 MOSFETs per arm and the -300, 6 per arm. With these assymetries 80% reduction could make sense for the -200 and 85% for the -300 controllers.

There are indeed applications other than ground vehicles, where reduced reverse current is less tolerable. So it has been decided to alter the current limits on the -200 and -300 controllers to reflect this reduced asymmetry.

The list below shows the suggested values and resulting currents for the three models. Ra and Ri can be identified from the drawing. Note that Ri can be two paralleled resistors: 2K2 and 22K to make 2K, 3K3 and 33K to make 3K. The two positions are shown as Ri(a) and Ri(b) and these are in parallel.

       No of MOSFETs
Model   H2    H1      Ra   Ri            Ifwd      Irev      Ratio 
                                                            
4QD-150 3     4       22K  3K3           170amps   120amps   68%  existing values
4QD-200 4     5       39K  3K (3K3//33K) 220amps   170amps   80%
4QD-300 6     7       47K  2K (2K2//22K) 330amps   270amps   85%

Control board layout (issue 13 shown)

The drawing shows the issue 13 control board: issues 14 and 15 are essentially the same here, although there is an empty space for a 22K resistor at top right.

Later Controllers

The 4QD series controllers now have symmetrical current limits - this actually simplifies the current limit circuitry as there are now only two versions of the control board;

1. All 36v controllers
2. All 48v controllers

36v controllers use SPP77N06S2-12 MOSFETS (typically R_ds(on) = 10mOhm) and 48v use SPP80N08S2-07 (R_ds(on) typically 5mOhm). A resistor change is required to accommodate this.

Resistor	36v	48v
Ri(a)	2K2	4K7
Ri(b)	note	note
Ra	open	open

Notes:
1: Ri(b) is usually omitted:
2: Ra is omitted: this makes the current limit symmetrical (forward limit = reverse limit). With symmetrical current limit, one MOSFET is (optionally) removed from Hi1, so all bridge legs have the same number of MOSFETs.

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