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.
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
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.
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.
IRL1104 has a
on-resistance of 9 mΩ
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
The 1N5819 is a 1W, 1A maximum
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
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.
Converter Design Details
The basic design of
the buck converter is described elsewhere for a similar circuit using a high-side
The losses in the
circuit come from the following sources:
inductor contributes ohmic losses from the windings and hysteresis and
eddy current losses from the core. These are unknown at this stage.
capacitor ESR contributes a loss due to ripple current. This is 0.3Ω x 0.2A / 6W = 1%
freewheeling diode contributes a loss due to forward voltage. The
through the diode is IO(1-D)
giving a relative loss of 3%.
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
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
The controller and
inverter contribute ohmic losses of their own during the on and off
way to obtain estimates of transient current flows is by means of a
simulation. This will also allow us to observe the dynamics of the
MOSFET turn-on phase.
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
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
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.
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
currents flowing through the 1K resistors in the controller and
useful set of reference documents is given here.