Expedite Transformer Calculations for Flybacks

Jan 1, 2008 12:00 PM
By Kirby Creel, Senior Design Engineer, Data-tronics, Romoland, Calif.

When charging defibrillator capacitors, a novel approach can be used with confidence to come up with a more exacting design, eliminating the cumbersome aspects of many other approaches.

Using a flyback topology to generate a high voltage is a common approach. The voltage can charge a capacitor for a high-energy pulse. Such an approach is used in defibrillator capacitors, photoflash capacitors, strobe capacitors and ignition circuits to name a few. Using a new step-by-step procedure, it's possible to quickly realizeaninitial flyback transformer design for charging a capacitor in a stated amount of time.

Following this procedure eliminates “cut and try” and over-design approaches. It also allows designers to select critical values with confidence, and can be used to provide insight as to the effects of holding an element constant and varying other elements. The variables included are frequency, voltage, pulse width, peak current, load capacitance and efficiency.

Before we delve into this novel approach, we need to understand the pros and cons of flyback topologies. Advantages include circuit simplicity, a high-voltage output that's not dependent on a large transformer turns ratio, a self-limiting circuit that can be short circuited without any damage, and an output that can be regulated over a large range. A flyback also can provide voltage isolation, allows for multiple isolated outputs where one output can be used for a low-ratio feedback voltage, and does not require a smoothing choke.

The disadvantages of flyback topologies include the need for a large and often bulky transformer, a fast-switching output that can generate problematic EMI signals, leakage inductance that must be kept low for good efficiency and a circuit that can be damaged once a load is removed without using a feedback loop.

Fig. 1 shows a simplified circuit and Fig. 2 shows idealized waveforms for a frequency of 50 kHz and a pulse on time (T_ON) of 45%.

The flyback operates by storing energy on the “charge” portion of the cycle and delivers the stored energy to the load on the discharge cycle. In the case of a flyback, the transformer is often described as a coupled inductor. Due to the diode polarity, current only flows in the secondary side during discharge. During the charge cycle, energy is stored in the primary inductance by a current ramp. The dead time shown in Fig. 2 ensures that the flyback is discontinuous. As the capacitor approaches full charge, the dead time increases.

The primary current ramp (charge) follows the inductance formula:

where L is the inductance in henries, V_A is the applied voltage, (Δt) is the time duration from start to finish of the applied pulse and (Δi) is the change in current over the same interval. If i starts at zero, Δi is equal to the peak current (I_PEAK):

Eq. 2 is Eq. 1 solved for I_PEAK. For example, if V_A = 12 V, Δt is 0 µs to 15 µs, L = 60 µH and I_PEAK = 3 A.

The energy stored in the inductance is:

where U is energy measured in Joules, L is in henries and I_PEAK is in amperes. In the previous example, the energy stored in each pulse is 270 µJ.

It is during the discharge of this stored energy that the greatest advantage of the flyback is realized. The output voltage will rise to whatever level is needed to cause current to flow, thus dissipating the stored energy. The voltage of the output has limits to be sure, but within the insulation structure, transistor breakdown, and circuit design, and taking losses into account, the voltage can rise to very high levels.

Though ancient technology now, the most common example was the flyback transformer in color TVs with a cathode ray tube. These transformers could generate voltages greater than 35,000 V, voltages so high that, when the circuits malfunctioned, the TV could generate damaging X-rays. The analysis is somewhat simplified because the TV flyback transformer performs more functions than just generating high voltage. The design of the flyback circuit and transformer for power transformation is well illustrated in Abraham I. Pressman's book Switching Power Supply Design.^[1]

The block in Fig. 1 labeled “pulse control” can take many forms. In a defibrillator, pulse control will be a voltage feedback loop that fixes the number of Joules to be delivered to the patient. During successive resuscitation attempts, the level will increase. For photoflashes, the charge level is fixed. The capacitor will be charged and additional pulses will only be applied as a refresh.

In photoflash applications, the dead time may be limited to speed up the charge time. The low dead time and variable discharge produces the characteristic of an increasing high-pitch sound. Variations in the pulse-control element are almost endless.