Buck converter uses low-side PWM IC

The most common switching-power topology is a buck converter, which efficiently transforms high voltages to low voltages.

 

 Figure 1 shows a typical buck converter in which the N-channel MOSFET, Q₁, needs a floating-gate drive signal. The floating-gate drive is part of the PWM (pulse-width-modulation) controller IC. Q₁ can be either N or P channel, depending on the controller’s design. Unfortunately, the IC’s voltage rating must be as high as the input voltage, which places a limit on the maximum voltage it can process.

 

The circuit in Figure 2 uses a simple voltage-level shifter that lets a buck converter control a pass transistor with a low-side IC that has a ground-referenced gate drive. Because the level-shifting circuit in the PWM IC does not have to tolerate high voltages, you can implement a converter with an arbitrarily high input voltage.

PWM ICs with low-side gate drivers can power N-channel MOSFETs that are on when they have a positive gate-to-source voltage. The circuit in Figure 2 uses a P-channel device as the high-side MOSFET; it’s on when its gate-to-source voltage is negative. Therefore, you must invert the control signal from the PWM controller. A MOSFET totem-pole configuration comprising Q₂ and Q₃ will work, although you can also use an inverting-gate driver.

Capacitor C₂ performs the level-shifting. It must have a value large enough to maintain its charge at the switching frequency but small enough for its voltage to follow variations in the input voltage. Resistor R₁ and P-channel MOSFET Q₃ charge C₂ to a voltage of V_C=V_IN–V_CC, where V_C is C₂’s voltage, V_IN is the input voltage, and V_CC is the supply voltage of the Q₂ and Q₃ totem-pole configuration and the PWM IC. The supply voltage must be less than zener diode D₂’s breakdown voltage. Otherwise, current will flow through D₂ and C₂ whenever Q₂ is on, which lowers efficiency. D₂limits C₂’s voltage to the value in the above equation. When Q₃ is on, D₂ becomes forward-biased if the voltage attempts to increase. This circuit applies a 0V voltage between Q₁’s gate and source when Q₃ is on, and it applies –V_CC when Q₂ is on.

 

Resistor R₁ also ensures that Q₁’s gate-to-source capacitance discharges, which keeps Q₁ off when the totem pole’s output voltage is high. Diode D₂ limits Q₁’s gate-to-source voltage to 12V regardless of the circuit’s input voltage. Capacitor C₂ is transparent to Q₁’s gate-drive pulse, so the circuit’s gate-driving capability is just as good as that of the totem-pole circuit itself. The level shifting, therefore, imposes no limitation on the size of the MOSFET that the circuit can drive.

Figure 3 shows a practical buck converter employing this scheme.

The converter’s input voltage is 18 to 45V, and its output voltage is 12V at a 1.5A output current. The converter uses National Semiconductor’s LM5020-1 flyback/boost/forward/SEPIC (single-ended-primary-inductance-converter) PWM-controller IC.

The figure retains the component designators from the previous figures but adds functions such as input-voltage filtering in C₉; input-undervoltage lockout in R₂ and R₇; soft-start capability in C₃; switching-frequency-setting ability in 12.7-kΩ R₃ for 500 kHz; feedback compensation in C₇, C₈, and R₆; and output-voltage setting in R₉ and R₁₀.

The LM5020-1 provides current-mode control, but, in this circuit, it implements voltage-mode control. An internal sawtooth-current source with a peak value of 50 μA, which adds slope compensation to a current signal, serves as a voltage ramp. This current flows through 5.11-kΩ resistor R₄ and an internal 2-kΩ resistor to generate a ramp with a peak-to-peak voltage of 50 μA×2 kΩ+5.11 kΩ)≈300 mV at the CS pin, Pin 8. The COMP pin, Pin 3, compares this sawtooth to the output error voltage at the COMP pin, which generates the right duty-ratio signal for Q₁.

Figure 4 shows the circuit’s switching waveforms.

 

Oscilloscope channel 1 (bottom trace) shows the gate-drive signal that the LM5020-1 generates. Channel 2 (middle trace) shows the corresponding totem-pole output voltage. Channel 3 (top trace) shows the level-shifted totem-pole output voltage between the source and the gate of Q₁. The peak value of Q₁’s gate-to-source voltage equals the input voltage, and its amplitude is about 8V, the value of the supply signal that the LM5020-1 internally generates. All the waveforms are clean and have short rise and fall times. The full-load efficiency of the circuit is 86 and 83% at input voltages of 18 and 45V, respectively.