Datasheet
SLUU087B
5
10-Watt Flyback Converter Using the UCC3809
2 Description
A brief description of the circuit elements follows:
D Transformer (a.k.a. flyback inductor) T1, transistor Q3, Schottky diode D3, input capacitor C18, and output
capacitors C10 through C12 form the power stage of the converter. Power resistor R7 senses the
primary-switch current and converts this current into a voltage to be sensed by the primary-side controller
feed-back comparator.
D Capacitor C14 filters out high-frequency noise on the output bus directly at the output diode.
D Resistor R11 and capacitor C7, along with resistor R10, capacitor C6, and diode D2 make up the voltage
snubber and clamp, respectively, for the primary side; likewise, resistor R12 and capacitor C8 provide
secondary-side snubbing.
D Resistor R8 supplies the start-up current to the primary-side controller, UCC3809. Operating current is
provided through the self-biasing (sometimes referred to as bootstrap) components D4 and C5.
D Resistor R6 and capacitor C1 filter out leading-edge current spikes which are caused by the reverse
recovery of the rectifier, equivalent capacitive loading on the secondary, and parasitic circuit inductances.
C1 should be returned directly to the IC ground.
D Resistor R24 and capacitor C13 provide leading-edge blanking to the voltage spike resulting from the
leakage inductance of the transformer. Failure to add these components would result in D4 and C5 peak
rectifying to this spike voltage.
D The primary-side controller functions are supported by external circuitry such as resistors R3 and R1,
combined with C9, which provide a charge and discharge path for the internal oscillator, setting the switching
frequency for the converter.
D Transistor Q4, resistors R20, and R22, along with dc voltage-blocking capacitor C15 provide slope
compensation.
D Capacitor C3 programs the soft-start time and transistor Q2, biased from the resistor divider R2 and R4,
is used to form an external shutdown.
D Capacitors C2 and C4 are decoupling capacitors which should always be good quality low ESR/ESL type
capacitors placed as close to the IC pins as possible and returned directly to the IC ground reference.
D The gate-drive circuitry is composed of gate-drive resistor R5 (necessary for damping any oscillations
resulting from the input capacitance of Q3 and any parasitic stray inductance) and pull-down resistor R13
(this resistor assures Q3 stays off in the event that U1 is removed from the circuit for any reason). Resistor
R18 and diode D1 are used to accelerate the turnoff time of Q3, while still limiting the gate current during
switch turnoff, thereby protecting the output stage of U1.
D The voltage-sense feedback loop is comprised of resistor-divider network R21, R17, and R16 (actually R21
provides a series 50-Ω injection point for small-signal control-loop analysis). Feedback components R15
and C16 provide the necessary gain and pole to stabilize the control loop, while R14 and R19 provide bias
current to optocoupler Q1, and secondary-side error amplifier and voltage reference Z1. Capacitor C17
helps to filter noise that may corrupt the exposed base terminal of the optocoupler while R9 provides the
proper offset for the voltage-feedback signal to be summed with the current-sense signal and the slope
compensation at the FB pin of the UCC3809.
3 Theory of Operation
When Q3 is turned on, T1 primary current increases linearly from zero. During this time, energy is stored in the
gap of the transformer while the load current is supplied by the output-capacitor bank, C10 through C12, as well
as C14. When this primary current has increased to a value at which the voltage across R7, summed with the
voltage sense and slope compensation, exceeds the FB threshold voltage of one volt, Q3 is turned off. The
stored energy in T1 is now transferred to the secondary side, forward biasing D3, and also replenishing the
charge on the output-capacitor bank.