Hi, I’m Roland van Roy.
I’d like to introduce the Richtek Load Transient Tool, a handy little tool that can be used
to quickly check the stability of DC/DC converters like Buck or Boost converters, Linear regulators
and even LED drivers.
You can check the stability of a voltage regulator by applying a fast load transient at the regulator
output.
The load transient generator consists of a MOSFET switch, a dynamic load resistor and
a static load resistor which is needed to keep the converter in continuous conduction
mode operation.
By measuring the load current and the converter output voltage, you can observe the load transient
behavior of the converter.
Transient response with smooth recovery indicates good control loop stability, which means sufficient
phase margin.
When a bump is observed, the phase margin is lower, but still OK.
When significant bumps are seen, the phase margin may become marginal, and severe ringing
in the waveform means low phase margin and poor control loop stability.
The Richtek Load Transient Tool is specially designed to generate fast load steps for easy
converter stability testing.
It has an on-board micro controller, a slew-rate controlled MOSFET switch, 7 selectable dynamic
load resistors and connection for a static load resistor.
Load pulse duty-cycle and frequency can be adjusted via push buttons, and a battery pack
on the bottom side makes this tool fully isolated and portable.
The Load transient Tool kit includes batteries and an adjustable power resistor for setting
the static load.
Let’s test the stability of a 3.3V supply which uses RT7297C, an 800kHz, 3A Current
Mode buck converter.
I used the Richtek Designer Online simulation tool to calculate the circuit values.The circuit
requires two 22uF output capacitors, and the design tool predicts good stability with a
control bandwidth of around 78kHz.
For the load transient stability test we first need to select the static load and pulse amplitude
of the load generator: I normally set around 30% static load and 30% dynamic load, which
keeps the regulator in continuous conduction mode.
So for a 3.3V 3A supply you would select around 3.6Ω static load resistor and 3.6Ω dynamic
load resistor.
First adjust the 3.6Ω static load via the adjustable power resistor, and then select
the 3.3Ω pulse load resistor via a jumper on the load transient tool.
Then measure the converter output voltage near the output capacitors and the current
in the load transient tool leads.
After switching on the load transient board you can see the buck converter transient response
on the oscilloscope.
The smooth waveform indicates good stability.
But you can learn more from this measurement: by measuring the step load response time of
a current mode buck converter which is the time from step-load start to the point where
the converter starts to recover, you can estimate the bandwidth or unity gain frequency of the
control loop.
In this case I measured around 4.6usec response time, and the bandwidth can be calculated
by 0.3 by 4.6usec which gives 65 kHz.
As we normally set the control bandwidth of a current mode buck converter below 10% of
the switching frequency the 65kHz measured bandwidth is a reasonable value for an 800kHz converter.
When I compared these results with an actual open loop gain-phase measurement using a network
analyzer, I measured a phase margin of 55 degrees and a control bandwidth of 60kHz.
So my simple measurement with the load transient tool was actually pretty close.
Current Mode Buck converter stability problems are often related to the converter output
capacitor value: Here is an example what happens when you change
the original two larger 22uF / 25V output capacitors into two smaller 22uF / 6.3V types.
You now see that the step response shows severe ringing which means low phase margin.
The converter response time has reduced to 2.6usec, so the converter bandwidth has increased
to around 115kHz which is really high for an 800kHz converter.
The reason for the poor stability is the reduction of the output capacitance: A 22uF/6.3V 0805
size MLCC capacitor has only 11uF at 3.3V DC bias, and will drop another 28% due to
its low AC ripple in a buck application.
So we end up with totally 16uF effective output capacitance instead of the original 44uF design
value.
This lower output capacitance increases the modulator pole frequency, and the open loop
unity gain frequency will increase to a point where the phase margin is lower.
This also explains why we measured higher bandwidth with the fast transient measurement.
To get stable performance again, you can either increase the output capacitance, or reduce
the compensator gain to reduce the unity gain frequency.
In this example, I chose to reduce the compensator gain by reducing the compensator resistor
in the same ratio as the output capacitance drop.
You now see stable performance again and the response time measurement shows the bandwidth
has dropped to 61kHz, which is a good target value for this converter.
I hope this example has given you an idea of what you can do with the Load Transient
Tool.
Please note that different converter types have different stability criteria.
The documentation that comes with the tool includes links to design tips and application
notes with more information how to check power converters using the load transient tool.
The Richtek Load Transient Tool kit is available at Farnell, Element 14 and Digikey.
If you are often working with DC/DC converters, I highly recommend to get this handy little
tool for your lab testing!