The Ridley Engineering Logo has meaning for anyone designing switching power supplies. In fact, you can generate this logo yourself in the lab, or with a circuit analysis program!
The logo is constructed of a family of curves showing what happens when you use current-mode or voltage-mode control for your converter. Or, as is frequently the case, something in between. The curves separating the colors of the logo were generated from measurements of the control-to-output transfer function of a buck converter operating at a 45% duty cycle and constant frequency. Current-mode control was used, with an adjustable amount of current feedback and an adjustable compensating ramp.
The top curve shows the transfer function gain you will measure if you don't use any compensating ramp in the control. The transfer function looks like a single-pole characteristic except for a sharp peak in the gain observed at half the switching frequency. This is caused by a pair of complex poles which represent the tendency of current-mode to oscillate as you approach a 50% duty cycle. If you don't add a compensating ramp, even at lower duty cycles than 50%, you may see this oscillation in your waveforms, especially once a voltage regulating loop is added.
The second curve shows what happens when you add a small amount of ramp to the control circuit-- the peaking of the double poles at half the switching frequency goes down, and the tendency to oscillate is damped.
As more and more ramp is added, the poles at half the switching frequency are even more damped, become real, and then separate. If you keep on going, adding more and more ramp relative to the natural slope of the current more like voltage-mode apparent.
There are several important design reminders in this logo:
- Don't assume you won't need a ramp if you stay below 50% duty cycle. A good threshold below which no ramp is needed would be 30%.
- Don't add too much ramp or your circuit will behave like a voltage- mode controlled system. This is a common mistake with current- mode control, especially in high-power converters which have very low ripple in the inductor.
- When you model a system, make sure that you use an adequate small-signal model which will give a three-pole representation of the current-mode system. A single-pole representation cannot model the system accurately enough.
If you are a user of POWER 4-5-6, you can generate the curves in this logo in a matter of seconds. Just change the compensating ramp height for a current-mode buck converter, and click on the Power Stage transfer function.