This note describes the operation of a "ZVS" (Zero Voltage Switching) driver used in a number of YouTube demonstrations for high voltage flyback and induction heating drivers. This note is the result of research I did on this circuit when investigating induction heating.
This circuit was introduced by Vladimiro Mozilli around 2009 and is often called the "Mazilli" driver. Although there are excellent tutorials and demonstrations on this driver on YouTube, the purpose of this note is to explain the basic operation of the ZVS circuit and to offer recommendations for its construction, operation and use.
The circuit initially was used as a high voltage driver for CRT flyback coils. Later, the same circuit was employed to drive induction heating coils.
This video shows tests on a 1000 Watt induction heater board.
This EBay listing shows a typical 1000 Watt induction heater.
Here is an interesting site that discusses the driver when used as a flyback driver. This same site also discusses the driver when used for a ZVS induction heater.
The ZVS Driver circuit is a Royer-type push-pull oscillator implemented with FETs. This type of oscillator can be used to drive the ferrite core of a flyback transformer to generate high voltage. It can also drive a high-current air-core "work" coil for an induction heater.
The high voltage driver was originally touted as an improved way of driving flyback cores. Until the introduction of Mazilli's circuit, flybacks were typically driven with a single transistor and a feedback winding or with a push-pull transistor configuration (e.g., using two 2N3055 transistors).
The operation of the circuit is descibed below, including its drawbacks and improvements. The operation also discusses selection of components, an area that is typically not well-covered at most sites.
It appears that component selection is not well understood because of the unique characteristics of a resonant Royer-type oscillator. The component stresses are considerable with this type of circuit because of the resonant operation of the primary which generates large voltages and currents that can stress poorly-selected components.
This section describes the basic operation of the ZVS Driver, including the design and purpose and selection of components. The descriptions below refer to Schematic 1
The ignoring the power supply chokes and resonating capacitor(s), the ZVS circuit is simply a push-pull FET-based oscillator where the drain of each FET is cross-coupled to the gate of the opposite (out of phase) FET.
The operation of this kind of driver for the primary of a transformer is very straightforward and need not be discussed here.
The circuit is fed from the power supply through a center-tapped primary coil and a single choke or from two separate chokes connected to the FET drains (as shown in Schematic 1).
With the two (47uH-200 uH) choke coils and the resonating capacitance across the primary, the circuit operates as a "Royer" type inverter where the primary is resonated with the capacitance and where the chokes keep the power supply from reducing the resonant action of the primary. These chokes also keep the primary oscillations from feeding back to the power supply. The role of these chokes and the resonating capacitor are discussed more fully below.
Zener diodes are connected across the FET gates so that the maximum gate to drain voltage is not exceeded. Use of a zener voltage higher than the minimum gate to source turn-on voltage ensures that the FETs are conducting at a high level.
The "fast diodes" connecting the drains to the opposite side FET's gate are typically Schottky diodes. These diodes provide a fast turn off of the FET gates when they are driven toward ground as the opposite-phase FET saturates. Although the forward-biased diode of zener can be used, it is significantly slower than the Schottky diode.
Thus, each FET is turned off as the opposite-phase FET turns on and this continues until the primary voltage reverses phase. This phase reversal happens when the primary voltage crosses zero so that switching occurs when there is zero voltage across the FET. Hence, "Zero Voltage Switching" or ZVS.
In some cases, the FETs are run without zeners to ground and the gate voltage is limited by a voltage regulator such as a 7812. With this configuration, the resistors from the power supply to the gate can be lower (e.g., 270 Ohms), thus reducing turn on time.
Because of the resonant voltage rise in the primary, the FET drains must withstand more than 3 times the power supply voltage. Most designs recommend a voltage rating of at least 4 times the supply voltage.
For a supply voltage of up to 30 or 40 Volts, the FETs should be rated conservatively at 200 Volts or higher.
Because of the resonant current rise in the primary, the FETs must be able to handle currents about 4 times the nominal power supply current. This requirement is normally satisfied by selecting FETs that have high continuous drain currents and Rds(on) resistances of less than a tenth of an Ohm.
A typical FET recommended for several designs is an IRFP260N rated at a voltage of 200 Volts and a drain current of 50 Amps. Drain "on" resistance is 0.04 Ohms and power dissipation is 300 Watts. The cost at Mouser is under $3.00 in small quantity. Here is the IRFP260N datasheet
Another consideration for the FETs is the gate to source capacitance (gate charge), which should be minimized to improve FET turn-on and minimize FET dissipation. With reasonably fast switching, dissipation requirements can normally be satisfied with relatively small heat sinks (e.g., because of fast switching and ZVS operation).
As noted earlier, the zener diodes limit the FET gate to source voltage to the maximum specified in the spec sheet. A 12 Volt zener, rated at at least one Watt, is commonly used in most ZVS circuits. The zener limiting resistors not only limit the maximum dissipation of the zeners but also drive the gate to source capacitance of the FETs.
For fast operation, the zener current limiting resistors should be as small as possible while not challenging the power rating of the zeners.
The resistors are typically tied to the power supply which can cause both zener and limiting resistor dissipation problems when increasing the supply voltage. Some designs use a separate gate power supply fed off the main power supply.
The cross-coupling diodes should be Schottky types with a voltage rating consistent with the power supply voltage and resonant rise considerations. Diodes rated at more than 200 Volts are commonly-available and can be "pulls" from old switching power supplies.
Some designs use the popular 1N5819 Shottky diode but its rating is only 40 Volts (use something else). One diode I found on a quick part search is a SCS306APC9. It is available from both Digikey and Mouser for under $3 (qty 10). It is rated at 650 Volts and 6 Amperes (available in a TO220 package).
Another fast diode in a TO220 package would be a FFPF30UA60S rated at 600 Volts and 30 Amperes. It is available from Mouser for about one dollar (qty 10).
The sample circuit diagram shows two chokes connected between the power supply and each leg of the primary. Some circuits use center-tapped primary and a single choke between the center tap and the power supply.
There seems to be some confusion about the chokes and their ratings. The impedance of the choke must be significantly higher than the impedance of the resonated driver primary but must also be able to handle the average current drawn from the power supply. If the choke is wound on a toroid or ferrite transformer core (as is common), then the core must not saturate at the maximum power supply current.
The wire used in the choke must also be able to easily handle the maximum current demanded of the power supply without significant resistive losses.
Some reported bad results are probably due to poor construction of these chokes. It is probably not a good practice to use randomly-selected toroids removed from old switchers without first testing their performance, especially with rated load current in the winding. It is probably best to separately test the chokes before deploying them or to closely follow a working design.
Many of the chokes seem to be built on toroid or ferrite transformers removed from ATX switching power supplies. Using "yellow core" iron powder toroids may require as many as 70 to 100 turns to get the appropriate inductance (around 100 uH for the induction heater).
It appears that using the core from a switching power supply transformer is a good approach. In one design, the chokes each consisted of 14 turns of hookup wire wound on a ferrite transformer removed from a switching power supply. The inductance was approximately 130 uH.
For an induction heater, the primary coil is typically a half-dozen or so turns of copper tubing. The primary coil corresponds to the primary of TR1 in Schematic 1. In some cases, water is run through the copper tubing with a small pump and plastic tubing to cool the coil.
For a high voltage driver, the coil is typically 8 turns of heavy insulated wire wound around the core of a flyback transformer.
The easiest case to discuss is the induction heater because the primary is an air core coil that will not saturate. This coil can also be a flat spiral-wound coil using copper tubing or Litz wire. This configuration is typically used with a flat metal plate held above the coil to heat water. The secondary can also be a washer directly placed in the water to be heated. The entire assembly, including the water to be heated and the metal washer, can be placed directly on the "work" coil.
For an induction heating coil, the inductance can be measured and the resonating capacitance selected, depending on the desired operating frequency. The operating frequency is often close to 100 kHz for commercial "1000 Watt" boards available for around $50 from EBay. Operation at frequencies above 1 MHz are achievable with low gate charge FETs and a properly-resonated primary coil.
When driving a ferrite core, it is important to make sure that the core does not saturate with the resonating current flowing in the primary. An noted previously, the resonating current is at least 3 times the current drawn from the power supply.
It is best to start with a known ferrite core type and calculate the saturation current from the number of turns and the power supply current. For some core types, the core can saturate at rather small power levels. In this case, a different core should be selected or a number of cores should be stacked together to get the desired power level.
A number of designs handle the saturation problem by inserting a small air-gap in the core. This is especially popular for generating high voltage with a flyback core when operated at high power and a rewound secondary.
The resonating capacitance is determined by using the primary inductance and target operating frequency plugged into the LC resonance formula.
The primary consideration for the resonating capacitance is that it be a high-quality capacitance capable of handling large AC currents. Normally, this means selecting a "MKS" type metallized polypropylene capacitor with a voltage rating at least as high as 4 times the power supply voltage.
Because of the very high circulation current through the resonating capacitance, it must be wired in close proximity to the primary coil and connected via low inductance circuit board traces or, in a pinch, braid. In most cases, several capacitors are used in a series/parallel combination. Using capacitors in series/parallel combinations reduce the current carried by each capacitor while allowing the adjustment of the resonating capacitance.
To test the selection and wiring of the resonating capacitor(s), the temperature of the capacitors can be checked after the circuit starts up. If the capacitors are getting hot, different capacitors and/or wiring should be used.
Good high-capacitance MKS capacitors can be pricey if purchased new (e.g., $2 each for 0.47 650 Volt units). Lower capacity and voltage units are less inexpensive, however (e.g., a 0.01 uF at 250 Volts capacitor for around $1.50 each). These capacitors also appear in old switching power supplies.
The biggest problem with the basic ZVS driver is that it can "lock up" if the supply voltage is not applied fast enough to kick it into operation. Several videos show this lock up and the high supply currents that result. The way to check for lockup is to monitor the primary for oscillation or to observe the power supply for large currents at startup. The recommended cure is to apply the voltage quickly, perhaps using a relay. It is common to see the power supply built with a microwave oven transformer that has had its secondary rewound for low voltage and high current. In this "brute force" case is is also common to see the power applied to the driver through a startup relay.
Schematic 2 shows a circuit that is said to prevent lockup and also corrects some other problems with the basic ZVS circuit.
The circuit in Schematic2 was downloaded from the medafire site.
The primary feature of the improved circuit is the use of a set-reset flip-flop to drive the FETs. Thus, each FET is assured of being driven by a phase that is opposite of the the other FET.
The improved circuit also drives the gates with a FET driver, ensuring that the gate charge is driven with a low-impedance source.
The RS flip flop gates are also driven from the two phases with voltage-protected inputs (i.e., using the protection diodes and 10K resistors on the gate inputs).
Another input of each gate is driven with an opto-isolator to allow the circuit to be turned on and off conveniently.
I have not run across a board that implements the improved circuit. Also, I have not tried this circuit or found any videos showing it in operation.
In doing further browsing for flyback drivers, I found that it is possible to drive a flyback with an electronic ballast for a fluorescent lamp. Ludic Science provides a good example of how to do this: Electronic Ballast Flyback Driver.
Ludic Science also describes how to connect two flyback coils in series: Cascading Two Flyback Transformers.
On Ebay, it is now possible to purchase (~ $20) a ZVS driver and a flyback transformer that have been connected to generate high voltage: ZVS & Flyback Driver on Ebay.
This is the test image (for the DECA external clock modification).
added electronic ballast addendum
corrected choke current requirements, misc. editing
rough formatting, schematics
Copyright © 2016, Bob Nash.