Emulated current mode control
The challenge of accurate and fast current measurement can be avoided with a new method that emulates the buck switch current without actually measuring the current. The switch current waveform can be broken down into two parts, a base or pedestal and a ramp. The pedestal represents the minimum (or valley) inductor current level. The inductor current falls to its minimum just before the buck switch turns on. A sample and hold measurement of the free-wheel diode current, taken just prior to the turn-on of the buck switch, can capture the pedestal current information.
The other part of the buck switch current waveform is the positive ramp to the peak level. When the switch is on, the inductor current slope is proportional to the input voltage and the inductor value. A similar voltage waveform can be emulated with a current source and a capacitor. If the current source is controlled in proportion to the input voltage, the capacitor charging slope is: dv/dt = K x Vin / CRamp , where K is a constant scale factor for the current source and CRamp is the emulation ramp capacitor. Each cycle when the buck switch is turned on, the capacitor voltage rises linearly. When the buck switch is turned off, the capacitor is discharged. The ramp capacitor voltage will be equivalent to the Buck switch current ramp, provided the ramp capacitor is selected in proportion to the value of the inductor.
The free-wheeling diode anode is connected through the controller. A small value current sense resistor and amplifier are used to measure the diode current. A sample and hold circuit captures the minimum level diode current just prior to the turn-on of the buck switch. Sampling the valley current each cycle provides the pedestal portion of the emulated current sense signal.
Once the sampled current pedestal and external ramp capacitor voltage are summed together this emulated signal can be applied to the PWM comparator. The regulator exhibits all of the characteristics of peak current mode control without the delay and transient effects in the current sensing signal.
The schematic of a demonstration circuit is shown in Figure 3. The peak switch current limit is internally set to 3A. However, the output current capability of this circuit varies with the input voltage. With low input voltage (10V) applied, the output current capability is approximately 1A. With high input voltage (50V) applied, the output current capability is approximately 2A.
Figure 3. Buck-Boost Regulator
About the authors
Robert Bell is the applications-engineering manager for National Semiconductor's Design Center (Phoenix), where he has worked for four years. The center's products include next-generation PWM power controllers, gate drivers, and hot-swap and load-share controllers. Before joining National Semiconductor, Bell designed power converters for military and space applications. In his spare time, he enjoys hiking, camping, tennis, and travel.
Kim Nielson is a Sr. Engineering Technician for National Semiconductor's Design Center (Phoenix), where he has worked for two years. The center's products include next-generation PWM power controllers, gate drivers, and hot-swap and load-share controllers. Before joining National Semiconductor, Nielson constructed prototypes and validated power converter designs for military and space applications. In his spare time, he enjoys golf, music, camping, and travel.