Electronic Design

Switch-Mode Control Tames Positive Motor Supply Voltage

This power supply is ideal for the circuitry that drives high-side p-channel MOSFET switches often found in motor controllers. It's especially useful for designs where the motor's positive supply voltage exceeds the typical 20-V gatesource breakdown voltage of the power p-channel MOSFET.

There are three notable advantages to the design:

  • It will always track the motor's positive supply voltage, which can be especially useful if the positive supply is one that changes value.
  • Unlike a boost converter, the circuit offers a true shutdown mode. Its output will just collapse close to the positive rail and then rebuild itself once the circuit is turned back on. When standby current drain is important, this trait can be very valuable.
  • When used as part of a pulse-widthmodulated (PWM) H-bridge with highside p-channel MOSFETs (compared to high-side n-channels), this power supply can achieve 100% duty cycles even at very low switching frequencies.

I chose to use ICs made by Linear Technology Corp. to take advantage of the company's free LTspice/Switcher CAD III simulator software. That Spice simulator allowed me to compare the performance of different design ideas before I built the working circuit. I ran transient simulations to determine optimum startup characteristics and transient load regulation at different motorsupply voltages. As a result, my circuit worked the first time when real hardware populated a real pc board.

Needless to say, there are many possible design alternatives involving many different boost converters, pulse-width modulation (PWM) controls, and difference amplifiers. Clever choices will permit motor supply operation beyond 100 V dc. For my design, I referenced the output of the power supply to 15 V below the motor's positive supply rail. That 15-V value is the voltage applied across the gate-source junction of the p-channel power MOSFET.

The circuit shown in Figure 1 employs the LTC1871 switch-mode controller (U1), the STN2NE10L n-channel MOSFET (Q1), and the LT1056 precision op amp (U2) as a difference amplifier. U2's output is attenuated by:

1/5 x (VM - VOUT)

where VM is the motor supply voltage. This allows U2 to operate from the lowest possible supply voltage. Because U1 also operates from the lowest supply voltage, the only components that see the full motor supply voltage are the Q1 MOSFET, the S100 Schottky diode (D1), the 22-µH load inductor (L1), the precision (0.1%) resistive divider input networks to U2, and any VM supply capacitors.

Here, the U1 controller operates in its burst mode. The key to the design is the circuit loop from the motor positive supply through the 33-µF load capacitor (C1), the L1 inductor, and back through the D1 Schottky. Connecting the Q1 MOSFET between ground and inductor L1 forms the series path through which L1 will charge.

Because the U2 amplifier compares the output voltage to the motor positive supply, it will detect when the output difference to the motor rail drops below the design value of 15 V. When this occurs, U1 turns Q1 on and charges inductor L1 from the motor positive supply through capacitor C1 until the current through Q1's 0.499-Ωsense resistor (R1) increases above 500 mA. Then Q1 shuts off, and L1 now discharges through D1 to the positive rail, charging C1 on the output.

The function of the 15-kΩ load resistor (R2) is critical because the input impedance of amplifier U2 to ground isn't infinite. The LT1056 amplifier's input resistance acts as a path to overcharge load capacitor C1. But the LTC1871 controller can't discharge this capacitor to bring the supply back to regulation. Load resistor R2 provides a discharge-current path, and it acts as a voltage divider to ground through VOUT when the supply is placed in standby mode. Standby is achieved by pulling the RUN pin of the LTC1871 to ground.

Difference amplifier U2 feeds back the output voltage compared to the motor supply rail, thereby setting the output voltage to 15 V below the motor supply rail. This 15-V difference is then attenuated through the amplifier and its attenuating resistors by a factor of 5 to 3 V. The resulting 3 V is then divided again by a resistive divider network to 1.23 V, which goes to the FB pin (comparator input) of the LTC1871. Changing the resistor divider network alters the 15-V design value. If you change this divider, remember that the output voltage of the LT1056 must remain within its specified limit.

Figure 2 shows a scope photo of measured results when the circuit is used as part of a PWM dc motor controller. In this case, the drive frequency is 20 kHz, and the duty cycle is set to 50%. Channel 1 (green) is the positive motor lead with the negative motor lead at ground, and Channel 2 (blue) is the output voltage of my switch-mode power supply, which is at 45 V. The peak voltage across the motor is 60 V.

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