dsPIC33CH Curiosity Development Board User’s Guide
DS50002762A-page 18
Advance Information
2018 Microchip Technology Inc.
2.5.5
Transient Load Tester Circuit
The MOSFET Q8 and surrounding components implement an adjustable
constant-current sink that can be periodically pulsed on for a few milliseconds at a time
to generate momentary SMPS output load transient pulses. During control loop firm-
ware development, it is often desirable to study the control system behavior in
response to large signal step changes.
By monitoring the SMPS output voltage waveforms in response to the load step
transient event, one can get an idea of the real world output voltage undershoot during
the transient and the subsequent overshoot that will occur after the transient load is
rapidly removed. Additionally, the transient response recovery waveform shapes can
also provide hints as to likely control loop stability and approximate phase margin.
Load step transient response curves exhibiting damped sinusoidal oscillating output
voltage, that takes a long time to recover to steady-state DC values, implies a control
loop with low phase margin, while an over damped RC-like recovery waveform implies
higher phase margin.
When the RC13_TRANSIENT logic signal is driven high, the MOSFET Q8 will begin to
turn on through the gate resistor R79. However, as the gate voltage rises, current will
begin to flow through the MOSFET and current sense resistor R94, which will create a
voltage that is sensed by Q9. When the voltage at the base of Q9 is sufficient to turn it
on, it will begin sinking current from the gate of Q8, preventing the gate voltage from
rising further and maintaining MOSFET Q8 in the linear region, where it behaves like a
voltage controlled constant-current sink.
Components, R83 and C40, provide compensation for the MOSFET Q8 gate waveform
to ensure small signal stable regulation of the constant current. The relative sizes of
R79 and R87 set the DC gain of the constant-current regulation control loop.
The value of current sense resistor R94 sets the current limit, but it is made adjustable
by biasing the base of Q9, up or down, via the resistor dividers R84 and R85. When the
S1PWM2L_DAC_ISET DC voltage level is high (e.g., near 3.3V), Q9 will always be
turned on, even with no current through R94 due to the resistor divider output (of
R84 + R85) being higher than the turn-on voltage of the BJT Q9. Conversely, when the
S1PWM2L_DAC_ISET DC voltage is low (e.g., near 0.0V), this decreases the voltage
appearing on the Q9 base, requiring larger currents through R94 before the MOSFET
Q8 gate voltage becomes limited.
Adjusting the PWM waveform duty cycle on RC5_S1PWM2L by +1.0% alters the Q8
constant-current sink value by approximately -12 mA. At 50% PWM duty cycle, the
approximate current sink level is around 390 mA, but will vary somewhat between
boards and at different ambient temperatures, as these will affect the Q9 turn-on
voltage. For exact current sink values, it is necessary to use closed-loop control by
measuring the RA2_TRANSIENTFB current sense voltage with the ADC at run time.
Then, using the resulting value to fine-tune adjust the PWM duty cycle on
RC5_S1PWM2L.
Since Q8 is driven in the linear region during the transient pulse, the instantaneous
power dissipation within the MOSFET can be quite high, potentially up to 15W if the
circuit is configured for 15V output and 1A pulse load current. This power dissipation
level cannot be sustained indefinitely without a substantial heat sink, but for short
pulses (ex:
≤
100 ms based on the safe operating area graph in the MCP87130T
MOSFET data sheet), the thermal inertia of the MOSFET die and package allow the
junction temperature to stay below the 150ºC maximum of the device. However, in
between pulses, enough time must be allowed for the die and package to cool back to
room temperature, before the next pulse, in order to ensure reliable operation of the
circuit. It is therefore recommended to control RC13_TRANSIENT, so as to generate
short pulses (ex:
≤
10 ms) with long off times between pulses (ex: pulse rate of ~5 Hz).