PeltierControllerOverview
Written 2016-01-03
Tags:Peltier SMPS PWM Fridge
Introduction To Peltier Controllers
A Peltier controller's job is to push electrical current through a Peltier junction(which acts as a heat pump), in order to achieve a specific temperature on one side of the Peltier. Peltier coolers have some interesting operating characteristics, so for controllers, there's a tradeoff between efficiency, cost, simplicity and lifetime.Tradeoffs
Peltier Self-heating
Outside of the Peltier effect, these devices heat themselves according to I2R, or current-squared time effective-resistance - the same as the power equation of a resistor. However, cooling is linearly related to current.Thermal Expansion
As peltier junctions heat or cool, the hot side expands and the cold side contracts. This thermal expansion creates mechanical stress, which will eventually destroy the device.Design Guidelines
Overdriving is inefficient
The ratio of useful work to wasted self-heat is improved if the Peltier is driven close to the target voltage, rather than toggled between max cooling and off.Minimize heat/cool cycles
Each peltier has a limited number of heat/cool cycles before mechanical failure.Existing Controller Designs
Overview
| Control System | Lifetime | Complexity | Cost | Efficiency Limiter |
| Bang-Bang | Shorter, due to thermal stress. | Simple | Cheap | Peltier Self Heating |
| Linear-Regulator | Long | Medium | Medium | Regulator efficiency |
| PWM | Long, if high enough frequency | Simple | Medium | Peltier Self Heating |
| DC-DC | Long | High | High | Regulator efficiency |
Bang-Bang
A is a simple controller consisting of some threshold triggers and bit of hysteresis between them. Below a certain temperature, one state is triggered. Above another temperature, a different state is triggered. This hysteresis prevents overly rapid on-and-off cycling of the peltier, but when engaged, the controller is running the peltiers faster than needed, and wasting power.Pulse-Width Modulation(PWM)
A PWM controller is related to a bang-bang, but operates on a much higher frequency, so that it can emulate an analog wave. The ratio of time on to time off over a period of time gives you the average voltage. However, these designs still tend to give the Peltier a varying voltage, and although thermal stresses are minimized as the frequency increases, filtering is needed on the output to minimize self-heating of the peltier junction.Linear Regulator
A linear regulator is useful to convert from a 12-volt supply to a lower output, varying according to how much heating or cooling is needed.
The peltier voltage is controlled by a comparator that scales the output acccording to the difference between the target temperature and the current temperature, and that difference fed to a transistor that limits the current through the peltier.
The lower voltage drives the Peltier efficiently, at the cost of the regulator's efficiency, which is directly related to the difference from the supply voltage to the output voltage (a 12v->3v has ~75% loss, a 12v->9v has ~25% loss).
DC-DC Controllers
The highest efficiency, but also most complex design, a DC-DC converter is used to produce an output voltage according to the amount required by a feedback circuit. This requires all the complexity of the linear regulator, except that the output transistor is replaced with a switched-mode power-supply(SMPS). One commonly available SMPS chip has an efficiency ranging from 67% to 93%, depending on input voltage, output voltage, and output current.Efficiencies
Modeling a 1 Ohm Peltier(to make the math simple) with a 12V nominal supply, and skipping over things like SMPS quiescent current, relay driver current, and fully-on transistor resistance:
| Controller\Load | 33%(4A Eq) | 66%(8A Eq) | 100%(12A Eq) | Loss Location |
| BangBang | 33% | 66% | 100% | Peltier |
| PWM | >33% | >66% | 100% | Peltier |
| Linear | 33% | 66% | 100% | Transistor |
| SMPS | 76% | 87% | 100% | DCDC |
By increasing the PWM frequency and filtering the output, it may be possible to get somewhere between PWM and SMPS efficiency, as the filter will smear the voltages from 12 and 0 to maybe 2 and 6 for a 4 amp average load, but the 6 amp peaks are still less efficient than SMPS.
Although the linear regulator has the same efficiency as the BangBang, we can easily improve the design by moving the transistor heatsink as far as possible from the Peltiers - with the BangBang and PWM, we are heating the cooling circuits, but with a Linear we can move that waste away from the fridge.