A Non-synchronous Buck dc-dc Converter, high side N-type MOSFET

This is a reasonably simple circuit for a moderate current dc-dc buck converter. The use of an n-type MOSFET requires slightly involved drive circuitry but allows use of a cheaper, more readily available n-type MOSFET with better characteristics than a p-type MOSFET.


Design Summary

The circuit design is described for a 12V to 6V dc-dc converter. The output current will be effectively constant at 1A. The output peak-peak ripple voltage is not critical but will be chosen to be 1% of the output voltage, that is 0.06V. The application is intended to power a 6V light from a 12V source.

The MC34063 output drives the gate of a small 2N7000 MOSFET acting as an inverter, since the polarity of the MC34063 will not drive the power MOSFET gate correctly. The inverter drain switches the power MOSFET gate through a totem pole driver. This driver includes a bootstrap circuit to ensure that the MOSFET gate is driven above its turn-on voltage during the on-period.

The IRL1104 n-type MOSFET is inexpensive although it is now not readily available. Other more readily available MOSFETS can be used, such as the IRF1404. The  IRL1104 has a typical on-resistance of 9 and a peak current of 104A. The on-resistance is much lower than is needed for this application but the device will be useful in other contexts. The gate charge needed to switch on the MOSFET to a gate-source voltage of 12V is about 45nC. This could be reduced to 30nC if a gate-source switching voltage of 8V were used. The gate charge tends to have an inverse relationship to the on-resistance and a different MOSFET choice may give a lower gate charge at the expense of on-resistance.

The 1N5819 is a 1W, 1A maximum average current Schottky diode which drops about 0.4V at 1A forward current. For the design the average current limit of the diode will restrict the current output to 2A.

C4 and C3 are low ESR capacitors. The 16V rated 47μF capacitor has a nominal ESR of 0.3Ω. C1 and C2 are decoupling capacitors for the controller and driver separately.

The inductor used for the circuit is a toroidal inductor measuring about 200μH and scrounged from an old computer monitor. No further detail is available at this stage concerning the core saturation characteristics or the ESR.

The measured efficiency of the circuit was 88%, giving 6.08V at 1.06A into a 5.6Ω load, and drawing 0.63A from an 11.5V bench supply.

Buck Converter Design Details

The basic design of the buck converter is described elsewhere for a similar circuit using a high-side p-type MOSFET.

Loss Computations

The losses in the circuit come from the following sources:

  • The inductor contributes ohmic losses from the windings and hysteresis and eddy current losses from the core. These are unknown at this stage.

  • The capacitor ESR contributes a loss due to ripple current. This is 0.3Ω x 0.2A / 6W = 1%

  • The freewheeling diode contributes a loss due to forward voltage. The average current through the diode is IO(1-D) giving a relative loss of 3%.

  • The MOSFET on-resistance contributes a small loss of about 0.2%.

  • The MOSFET turn-on transient comes from the charge needed to turn on the device. In the turn-off period this charge is delivered to the output rather than back to the source, and as such most of it will not be counted as loss. Similarly the bootstrap capacitor recharges at the beginning of the off period. Small losses can occur in the BJTs and also in the bootstrap diode due to these currents. With careful layout the heavy current flows associated with charging the bootstrap capacitor and the MOSFET gate can be localized around the MOSFET, thus reducing the effects of stray inductances and resistances.

  • The controller and inverter contribute ohmic losses of their own during the on and off periods.

The quickest way to obtain estimates of transient current flows is by means of a SPICE simulation. This will also allow us to observe the dynamics of the MOSFET turn-on phase.

The currents through V2 (inverter and driver) and V3 (inverter) are shown in the first graph below. These currents flow downwards in the circuit. The first graph shows the currents in the inverter during the on-period. The large spike through V2 at the end of the on-period is due to the charging of the bootstrap capacitor. There is also a pulse through V3 which is due to shoot-through current flowing through the MOSFET totem pole driver upper BJT and the 2N7000, between the time that the 2N7000 turns on and the BJT turns off. This is limited by the 100Ω resistor R3. The loss due to this current pulse can be reduced by increasing R3. The second graph shows the current passing to the output from the driver as a result of the charge and discharge of the MOSFET gate. The pulse at the end of the on-period is due to the discharging of the MOSFET gate and the spike is due to the charging of the bootstrap capacitor (matching the spike in the current through V2). The negative going pulse at the start of the on-period occurs as the bootstrap capacitor charges the MOSFET gate. Part of this is reflected in a small pulse through V2.

The losses due to these currents will occur in the diode and the BJTs during the brief turn-on and turn-off times. Losses can occur in the resistor R3 due to the shoot-through currents. A loss also occurs during the on and off periods due to steady currents flowing through the 1K resistors in the controller and inverter.


References

A useful set of reference documents is given here.


First created 5 August 2010

Last Modified 11 August 2010
© Ken Sarkies 2010