Appendix

Chart 1. DC–DC Design Chart

Chart 2. AC–DC Design Chart

CCM Assumed	Single-Ended Forward (like a Buck)	Flyback (like a Buck-Boost)
Transformer turns ratio “n”
Reflected output voltage “VOR”
Reflected input voltage “VINR”
Reflected output current “IOR”
Reflected input current “IINR”
	(See Figure 3.2) See also, the equation for IIN further below
Duty cycle
	(See Figure 5.1)
Ideal duty cycle “DIDEAL”
DC transfer function “VO/VIN”
	η is the efficiency of the converter=PO/PIN, D is the actual/measured duty cycle , and n is the turns ratio.
Inductance “L” (μH)
	(See Figures 5.8, 5.9, and 14.4) This refers to the inductance of the output choke of a Forward converter and the Primary side of the transformer in a flyback (measured with the Secondary windings open). f is the switching frequency in Hz, and r is the current ripple ratio; see below. Typically, choose L such that r=0.4 (i.e., inductor current swing is ±20% of its DC or center of ramp value IL); also, set r to this value at the highest input voltage for Forward and at the lowest input voltage for flyback.
Average current in inductor “IL” (or center of ramp)		Primary side:
		Secondary side:
		(See Figure 3.2)
Current ripple ratio “r”
	(See Figures 2.2 and 3.2) r is the current ripple ratio=ΔIL/IL≡2×IAC/IDC, where IL is the average inductor current (the center of ramp, i.e., IDC) and IAC is its AC component≡ΔIL/2; L is in μH and f is in Hz.Typically, set r=0.4, that is, inductor current swing is then ±20% of its DC (center of ramp) value “IL.” For a flyback, r is the same on either side of the transformer, though currents and current swings on either side of the transformer are scaled as per the turns ratio.
Peak-to-Peak current in inductor “ΔIL”
	(See equations for IL above)
Peak-to-Peak current in input capacitor
	Ignoring transformer magnetization current
Peak-to-Peak current in output capacitor
Input voltage ripple (p–p) component (ESR-related)
	Ignoring transformer magnetization current
Output voltage ripple (p–p) component (ESR-related)
Input voltage ripple (p–p) component (capacitance-related)
	Ignoring transformer magnetization current
	(See Figures 13.1–13.3)
Output voltage ripple (p–p) component (capacitance-related)
	(See Figures 13.1–13.3)
RMS current in input cap
	Ignoring transformer magnetization current
	(See Figures 7.6 and 7.7) For a Forward converter, max RMS current in CIN occurs at D=0.5, that is, at which VIN/η=2×VO.
RMS current in output cap
RMS current in inductor and windings	Primary side:	Primary side:
	Ignoring transformer magnetization current
	Secondary side:	Secondary side:
	Output choke:
	(See Figure 7.3)
RMS current in switch
	Ignoring transformer magnetization current
RMS current in diode (or sync FET)	Output diode (to transformer):
	Freewheeling diode (to ground):
	(See Figures 7.3 and 7.6)
Average current in switch
	Ignoring transformer magnetization current
Average current in diode	Output diode (to transformer):
	Freewheeling diode (to ground):
Average input current “IIN”	Same as average switch current
Peak energy handling capability of core “ε” (μJ)
	(See Figures 5.5 and 5.6) This peak energy refers to the output choke of a Forward converter and to the transformer of a flyback. For flyback, use the Vμs appearing across the Primary winding, that is, VIN × D/f × 106 or equivalently VOR×(1−D)/f ×106.

Chart 3. Multi-topology Voltage Stresses Design Chart

See additional comments in Table 7.1.