The TinySwitch-III supply shown in Figure 1 provides 20 W of
output power. The 15 V output is capable of delivering up to
50 mA of current and is typically used to supply the primary side
driver for a brushless DC (BLDC) motor as it is referenced to the
source of the MOSFET internal to U1 (primary side). The 12 V
regulated isolated output, delivering up to 1.6 A, CAN be used to
drive a Microcontroller relays, and a stepper motor.
The supply uses a large input capacitor, C1, to maintain a
constant voltage source for the BLDC motor. It is assumed that
this supply will be placed after all necessary EMI ﬁ ltering and AC
rectiﬁ cation, and thus none of these components are shown.
However, Y-capacitor C10 is shown as part of the EMI ﬁ ltering that
may be necessary.
The TNY279PN device is used in a ﬂ yback topology. During the on
time of U1’s internal MOSFET current rises linearly in the primary
winding of transformer T1. While current builds in the primary,
energy is stored in the transformer. Once the current through the
MOSFET reaches the device’s current limit, or DCMAX is reached, the
MOSFET turns off. The ﬂ yback portion of the conduction cycle now
begins. During this period a voltage is induced in the secondary
winding, and the energy stored in the transformer is transferred to
the output capacitors and the load.
During the supply’s ﬂ yback period, an RCD snubber circuit keeps the
voltage at the drain node below the device’s BVDSS limit. The snubber
circuit is comprised of fast-blocking Diode D1, capacitor C2, and