The low-power (milliwatt) flyback transformer used in a switched-mode power supply (SMPS) needs to be designed with a gap in the magnetic path for maximum power delivery over temperature. Transformers built on ferrite structures are typically gapped for higher-power storage capability or for large dc currents.
In low-power or low-dc-current applications, where a machined gap isn’t needed for power or current handling, there still needs to be a gap to stabilize the ferrite’s temperature variation characteristics and provide improved power delivery to the load.
The flyback transformer is very different from a signal transformer, and not making the distinction can lead to poor performance. Actually, the flyback transformer is a coupled inductor and not a transformer in the true sense.
In the signal transformer, current flows in the primary and secondary windings at the same time, inversely proportional to the turns ratio of the windings. In a flyback transformer, current flow is restricted to one winding at a time.
Designing a flyback transformer using a ferrite core that doesn’t have a gap might seem appealing for low-power (milliwatt range), low-current applications. But don’t fall into this trap because it will cause poor performance. The power delivery of the SMPS using this ungapped transformer will be limited.
This limitation is not apparent at room temperature, normal input voltages, or normal load demand. But it will become obvious at high temperature with low input voltage or increased load demand. Thus, an important aspect of flyback transformer design is to “mind the gap.”
SMPS OPERATION
Figure 1 shows a simplified typical SMPS converter using a flyback transformer. A source voltage is switched on and off, by means of a transistor (MOSFET), drawing current through the flyback transformer’s primary.
When the transistor is on (tON), the current through the primary windings ramps up in proportion to the applied voltage and inversely to the value of the primary inductance. Because of the blocking diode and the polarity of the secondary winding, there is no current flow in the secondary winding at this time.
When the transistor opens (tOFF), the primary winding current drops to zero, causing the voltages on all of the windings to reverse in polarity. With the secondary polarity reversed, the secondary current can now flow through the forward biased diode. The energy stored in the core transfers to the secondary windings and into a charging output capacitor and the load.
For the SMPS circuit in Figure 1, a typical current through the primary winding is expressed via a well-known electrical engineering formula using the inductance, voltage, and current relationship:
The voltage (V) across an inductor winding produces a ramping current (di/dt) with respect to the inductance value (L). Figure 2 shows some typical waveforms from a flyback SMPS circuit (such as the one shown in Figure 1).
Peak current in the primary winding is the magnitude that the current reaches at the end of the transistor time on (tON). Figure 2 shows this as the maximum value of current (ILp) (after ramping up at the slope value of di/dt) as expressed by Equation 1.
When a constant value of voltage is applied across an inductor winding, the current ramping will be steeper (ramps faster) for a lower value of inductance and flatter (ramps slower) for a higher value of inductance. Inductance value and ramping current are inversely proportional.
High values of di/dt (low inductance) result in added ripple currents that need to be compensated for or filtered out. However, low values of di/dt (high inductance) result in less energy stored and transferred to the load (since energy is directly related to the square of the current). These dynamic situations can be compensated for with feedback to alter the pulse width to the switching transistor, but there is a limit due to the switching frequency.
In pulse-width mode (PWM), the (tON) period is manipulated by a pulse-width controller device, which sends a signal to the gate of the MOSFET in Figure 1, based on feedback circuitry from the load. In pulse-frequency mode (PFM), the frequency of the switching is altered to accommodate changes in load demand. The feedback to the control circuit is from load voltage or current sensing devices.
These control circuits are designed around the assumption that the inductance of the flyback transformer is within specified values. When the inductance value varies beyond the specified limit, the power delivering capability of the SMPS suffers.