MC34063 Analysis and SPICE Model

Here is a set of notes on the MC34063 smps controller chip. This chip has been around for quite a while and is very inexpensive. It can provide switched power conversion at currents of a few hundred milliamps to over an amp depending on the configuration. It comes in an 8 pin DIP and requires only a handful of additional components. It is useful in applications such as providing analogue positive and negative supplies from a single 5V logic supply in small circuit boards. The control functions provided consist simply of a comparator that gives hysteretic or bang-bang control. This is thought to be very effective, giving excellent transient performance, but its stability is not well understood. The aim of these notes is to provide some insight into the operation of the chip and associated circuits.

  1. Theory of operation

  2. SPICE models

  3. Practical circuits and problems experienced

  4. Resources

Theory

The theory of operation is well covered in reference 2 of the resources. The circuit operates by charging a timing capacitor on pin 3 with a 35µA current until the voltage between it and ground (pin 4) reaches 1.25V, then discharging at a current of 200µA until the voltage falls to 0.75V. The charge current is modified by a voltage at the Ipk-sense pin to shorten the charge time and so reduce the maximum duty cycle. This provides a form of current limiting. A set-reset flip flop is held in the reset state during the discharge cycle, causing the output switching transistor to turn off. During the charging period the flip flop is set when the voltage at the feedback input (pin 5) falls below 1.25V. If the feedback voltage remains above 1.25V then the flip flop remains always reset, while if the feedback voltage remains below 1.25V the flip flop is set for about 5/6 of an oscillator period, and reset for the remaining 1/6 of the period (except during current overload).

SPICE Models

For simulation of analogue circuits, Berkeley SPICE has been the program of choice for over 30 years. This program is opensource and has been rewritten many times to improve performance and to adapt it to a number of specialist application areas. While it is no replacement for measurements on hardware, it can provide quite good behavioural modelling of complex circuits, allowing many errors and problems to be corrected before the hardware is built. It is also valuable for gaining an intuition into how circuits work.

Two opensource simulators are provided with the gEDA design suite: ngspice and gnucap. The former is developed around the Berkeley SPICE 3 simulator, while the latter is interactive, provides mixed mode simulation and has SPICE compatible models.

A model for the MC34063 was developed in 2002 by AEi Systems (reference 3) for the Intusoft simulator. This uses syntax for behavioural modelling that is different to that of SPICE 3. The conversion to PSpice mentioned in reference 3 uses different syntax again.

In the following a SPICE 3 compatible model for the MC34063 is provided. The model is different to the AEi model but makes use of some information about MC34063 behaviour. It is provided as opensource under the GPLv2 licence. The model uses the u() step function of SPICE 3 to provide digital signal conversions and processing, and as such should work with simulators that claim SPICE 3 compatibility.

The following diagram of the model is presented in terms of comparators and standard digital gates for clarity.


The gschem circuit shown above is given here for illustrative purposes. It includes a number of simple logic gates built from nonlinear dependent voltage sources.
This file is not suitable in itself for generating a subcircuit as the gates need to have their supplies referenced to the common pin of the MC34063 rather than to ground (this could be done by defining custom symbols). A well documented subcircuit file is given here. This has been tested using a number of test circuits including the buck, boost and buck-boost example configurations shown in the datasheets. The above circuit can be run as a simulation. Simply place all files in the same directory and move the symbol files to the gEDA sym/local directory.

The circuit in the top left of the diagram above is the oscillator. This consists of a comparator working with hysteresis to provide the 1.25V/0.75V switching points. To improve performance this was replaced with a controlled hysteresis switch similar to that used in the AEi model. The oscillator-source block generates the timing capacitor current in response to the output of the comparator. This current is modified by the transistor attached to the Ipk input to cause the capacitor to charge up faster if Ipk increases above about 400mV. This provides the current limiting capabilities of the device. The signal "oscillator" provides a pulse that goes low when the capacitor is being discharged.

In the top right of the circuit is the flip flop that drives the switching transistor. The differentiator formed by Cdelay and Rdelay along with the squaring comparators provide two short pulses, one following the leading edge of the oscillator pulse (delayminus) and one following
the trailing edge (delayplus). These are used with the gates Xminus and Xplus to ensure that the reset pulse is shortened at its leading edge, and the set pulse is shortened at its leading edge. This is needed to prevent the set and reset inputs of the flip flop being active at the same time, and so causing the simulator to choke. The RC integrators in the flip flop are also needed for the same reason. The time constant of these must be much shorter than the width of the delay pulses.

In the lower centre of the circuit is the comparator that compares the feedback input with the reference voltage to gate the set pulses to the flip flop. The low-pass filter at this point was necessary to ensure convergence. Xoutgate ensures that the flip flop output signal is turned off when the feedback control signal is off. The switch circuit includes a drive transistor fed by a current source that ensures it is firmly turned on and off in response to the gated control signal. The transistors need to have some capacitances and resistances to be specified to avoid convergence problems.

Testing

It is assumed that a working version of gEDA and ngspice is available with a set of suitable SPICE models installed in the gEDA/models directory. The latter can be obtained using the method described in the gEDA setup page. To get a basic simulation going try the following commands. Ignore any error messages about unrecognized parameters in the diode model.

$ gnetlist -g spice-sdb buck.sch -o buck.net
$ ngspice buck.net
ngspice 1 -> tran 100ns 10ms uic
ngspice 2 -> plot out
ngspice 3 -> plot x1.cinvcontrol x1.set x1.reset

The former plot should show the circuit output hovering around 5V with a startup transient. The second will show the gating signal applied to the set signal of the flip flop and the set/reset signals in operation. Right-click on the graphs and drag a small box to zoom in on the detail. This is about all that can be done with these graphs.

Note that the uic parameter in the transient analysis command, which says "use initial conditions", is eseential to allow convergence to occur. The reason for this is that at least one capacitor inside the MC34063 model has initial conditions. Beware that GUI helpers like easy_spice may not include this parameter.

Practical Circuits

The focus of this page is to study the behaviour of some switching power circuits using the MC34063. For designing dc-dc converters, the datasheets have a table of formulas that may be used, however this should not be a replacement for understanding of the circuit operation as the designs may need to be tweaked for a variety of reasons. As well as the spreadsheet provided here, more comprehensive resources can be found on the web, see in particular references 4 and 5.

On a practical note, take extra care with board layout. These chips can be sensitive to noise on the power input rails, resulting in the flip flop triggering prematurely and resulting in poor regulation and overvoltage. Ceramic capacitors in parallel with the power input electrolytic capacitor should be used to attenuate high frequency switching noise.

Buck Converter


This is a straightforward dc-dc converter that produces an output voltage less than the input voltage. We consider the conversion from a 12V input to a 5V output for driving logic circuitry.


This circuit uses a symbol from the gEDA symbol collection for the MC34063. The 0.3R resistor provides nominal current limiting at about 1A, although it kicks in seriously at about 1.5A with the model we are using for the current limiting BJT. This is a bit lower than that for the AEi model and closer to the 300mV trigger specified for the MC34063. The 3R load allows a clear demonstration of the current limiting in action. A 100µF filter capacitor is a bit small for this circuit in practice, but allows a transient simulation to be run in a reasonable time.

The following plot shows a detailed portion of the resulting simulation in ngspice. The blue plot is the voltage across the current limiting resistor R0, which is proportional mainly to the current through the output switch BJT. The yellow plot is the feedback comparator gating signal that turns the switching process
on and off. With a large filter capacitor, this signal typically spans a number of oscillator cycles. The upward slope of the switch current is due to the ramping up of the inductor current.

This represents a circuit near to the current limiting point, having a current of 1.4A (any higher and the output voltage drops away while the feedback gating signal is always on). Examination of the red oscillator trace shows that the charge time is shorter during a gated on period. The changes in oscillator slope and in the cycle period are clearly visible as the feedback gating signal turns on and off.


A computation of the mean power in the source and in the load gives a 95% efficiency at 50mA load current. Measurements give 83%. Some of the difference may be in the filter capacitor ESR and inductor series resistance which we have assumed as zero.

Resources

  1. Data Sheet from ON Semiconductor.

  2. AN920/D ON Semiconductor publication "Theory and Applications of the MC34063 and µA78S40 Switching Regulator Circuits".

  3. Application of the MC34063 Switching Regulator Shafi Sekander and Mahmoud Harmouch , Texas Instruments 2007.

  4. DC-DC Step-Up/Down Converter used to Design a Switching Power Supply, Adriana Florescu, Constantin Radoi.

  5. Intusoft SPICE and PSpice models for the MC34063 from the book by Christoph Basso.

  6. Lazar's Power Electronics Corner. Lazar Rosenblat.

  7. Switching Mode Power Supply Design. Jerrold Foutz.


First created 14 August
2007
Last Modified 17 March 2014
© Ken Sarkies 2007