Enhance Power Supply Efficiency Through Improved Thermal Management

Technical Overview: Rising power density increases heat from switching transistors, diodes, magnetics, capacitors, and integrated circuits in switched-mode power supplies (SMPS). This article explains how conduction and switching losses, diode forward voltage, winding and core losses, capacitor equivalent series resistance (ESR), and silicon power dissipation contribute to thermal load. It compares natural convection, forced air, conduction, and liquid cooling, including their capacity, reliability, noise, maintenance, and environmental tradeoffs. It also covers thermal budgeting, printed circuit board (pc board) layout, heatsink selection, airflow management, simulation, and worst-case testing. Finally, it examines the Analog Devices MAX31785 six-channel intelligent fan controller and ADN8834 thermoelectric cooler (TEC) controller for dynamic fan regulation and precision temperature control. (Overview courtesy of ChatGPT).

As demand grows for smaller, more capable designs, accumulated losses in switching transistors, magnetics, capacitors, and integrated circuits create thermal stress that degrades performance, shortens component lifetimes, and risks outright failure.

Designers need methods to better manage these heat sources, balance cooling tradeoffs, and use dedicated controllers to precisely and reliably regulate fans and thermoelectric coolers (TECs).

This article reviews the thermal challenges facing power supply designers. It then shows how to address these challenges using dedicated devices from Analog Devices.

Identifying heat sources in power supplies

Excessive heat degrades key measures of power supply performance, lowering conversion efficiency, shortening mean time between failures (MTBF), and limiting achievable power density. As designers pack more power into less volume, thermal management becomes a critical design challenge.

Effective thermal management begins with quantifying the heat sources in each major component class of a switched-mode power supply (SMPS). Devices such as metal-oxide-semiconductor field-effect transistors (MOSFETs) and diodes, as well as magnetic components such as transformers and inductors, generate heat through well-defined mechanisms, adding to the heat contributed by resistive elements and control circuitry.

MOSFET switching transistors dissipate power through both conduction losses and switching losses. Conduction losses scale with the square of root-mean-square current (I²_RMS) flowing through the MOSFET’s drain-source ON resistance (R_DS(ON)) (Equation 1).

 Equation 1

Switching losses occur during transitions and increase with higher frequency and current (Equation 2), making MOSFETs one of the leading sources of heat in SMPS designs.

 Equation 2

Where:

V_ds = drain-to-source voltage

I_d = drain current

t_on, t_off = switching transition times

f_sw = switching frequency

0.5 = assumption of linear overlap of voltage and current during switching

Diodes generate heat (P_diode) due to their forward voltage drop as current flows through them (Equation 3). At high switching speeds, reverse recovery adds further loss and associated incremental heat.

 Equation 3

Where:

V_f = forward voltage drop (0.7 V for silicon (Si), 0.3 V for Schottky)

I_avg = average current through diode

Heat from magnetic components arises from copper losses in the winding resistance (P_cu), which increase with current (Equation 4), and core losses (P_core) from hysteresis and eddy currents, which rise with frequency and flux density (Equation 5) (Steinmetz equation).

 Equation 4

Where:

I_rms = rms current in winding

R_winding = DC resistance of winding

 Equation 5

Where:

k = Steinmetz coefficient (material specific)

f = frequency (Hz)

B = peak flux density (Tesla)

α β = Steinmetz exponents (typically α ≈ 1.3 - 1.6, β ≈ 2.0 - 2.8)

V_core = core volume (centimeters cubed (cm³) or meters cubed (m³))

With capacitors, heat arises due to ripple current (I_ripple) through their equivalent series resistance (ESR), particularly in electrolytic capacitors, resulting in localized heating (P_ESR) (Equation 6) in power supply filters.

 Equation 6

Adding to the thermal load of power supply designs, heat is generated by electrical power dissipation (P_d) within the silicon, which subsequently causes the junction temperature (T_j) to rise above ambient (Equation 7).

 Equation 7

Where:

T_j = junction temperature (°C)

T_a = ambient temperature (°C)

θ_ja = thermal resistance from junction to ambient (°C per watt (°C/W))

P_d = power dissipation (watts)

Summing these losses across the design yields the total heat load. Careful component selection and circuit design reduce that load at its source, and a well-chosen cooling method removes the remainder.

Comparing power supply cooling methods and their tradeoffs

Once the heat load is determined, power supply designers rely on four principal cooling methods, each trading heat-removal capacity against complexity, reliability, and environmental fit.

The simplest approach is natural convection, which relies on buoyancy-driven airflow to remove heat from components and enclosures (Figure 1). Because it uses no moving parts, natural convection operates silently and avoids the cost, power consumption, and maintenance associated with fans. Heatsinks are often added to increase the surface area available for heat dissipation. Even so, heat-removal capacity remains limited, and effectiveness depends on ambient conditions and enclosure orientation.

Figure 1: Natural convection cooling relies on buoyancy-driven airflow, providing silent, highly reliable heat removal with no moving parts but limited capacity. (Image source: Analog Devices)

When natural airflow cannot keep pace with the heat load, forced-air cooling uses fans or blowers to increase convective heat transfer well beyond natural levels (Figure 2). When designed with effective fan speed control and monitoring, this approach scales with the load and works well in compact designs. In return, designers must accept mechanical wear and maintenance, audible noise, and sensitivity to dust and humidity.

Figure 2: Fans and blowers increase convective heat transfer well beyond natural convection levels, but at the cost of mechanical wear, noise, and maintenance. (Image source: Analog Devices)

Conduction cooling takes a different path, using thermal interface materials to transfer heat directly to a heatsink or chassis (Figure 3). Performance depends on the path's thermal resistance, set by the conductor material and the contact area available for heat transfer. With no moving parts, conduction cooling is rugged and well suited to sealed enclosures and harsh environments, although its performance depends on careful mechanical design and on the ability of the chassis or external cooling structure to dissipate the transferred heat.

Figure 3: Conduction cooling moves heat directly into a heatsink or chassis through thermal interface materials, suiting sealed enclosures and harsh environments. (Image source: Analog Devices)

Suitable for heavy thermal loads, liquid cooling circulates coolant through cold plates to transfer heat to radiators for dissipation (Figure 4). The superior thermal conductivity and heat capacity of liquids enable the highest power densities while reducing fan noise. These benefits come at the cost of pumps, tubing, leak risk, and the ongoing maintenance required to ensure optimal thermal performance.

Figure 4: Circulating coolant through cold plates and radiators supports the highest power densities but adds pumps, tubing, cost, and leak risk. (Image source: Analog Devices)

For most designs, the choice comes down to how much heat must be removed and what the operating environment allows. Sound design practice then determines the effectiveness of the chosen thermal management method.

Applying thermal design best practices

Thermal design begins with a thermal budget that allocates the allowable temperature rise across the design, supported by power profiling that calculates component power dissipation, including switching, conduction, and magnetic losses, under worst-case operating conditions.

For the printed circuit board (pc board), a thermal-aware layout design uses copper planes that spread heat laterally away from hot components and thermal vias in multilayer boards that conduct heat vertically to inner layers or to the opposite side of the board (Figure 5). Guided by power profiling results, designers place high-power components near heatsinks or other thermal paths to shorten the distance heat must travel, while locating heat-sensitive electronics away from heat sources to mitigate thermal effects.

Figure 5: Copper planes spread heat laterally while thermal vias conduct it through the board, turning the pc board into an effective heat spreader. (Image source: Analog Devices)

Optimal heatsink selection and airflow management are critical for removing heat from components. Here, designers match heatsink thermal resistance to the dissipated power and available airflow, while selecting design-specific characteristics, including fin geometry, orientation, and surface treatments such as anodizing that further enhance heat transfer. Airflow management ensures that cooling air reaches the components that need it, starting with properly sized and positioned intake and exhaust vents. Computational fluid dynamics (CFD) analysis then identifies recirculation zones and dead spots before hardware implementation.

Active cooling performs best with dynamic control methods that respond to changing thermal loads by adjusting fan speed via pulse-width modulation (PWM) or voltage control, using temperature feedback. Thermal alarms and fault detection add an important safety element, alerting the system during over-temperature conditions or when a fan degrades or fails.

Finally, thermal design best practices also include careful consideration of the operating conditions the design will encounter in the field. Convective heat transfer depends on air density, which varies with altitude, temperature, and humidity. High-altitude or high-temperature operation may require derating components to account for degraded heat transfer. In harsh environments, conformal coatings and sealed enclosures may be needed to protect sensitive designs.

Avoiding common thermal design pitfalls

The most common thermal design failures typically stem from the ineffective use of these best practices. Chief among them is underestimating worst-case heat generation. A design profiled only under typical loads can overheat when maximum load, high ambient temperature, and reduced cooling coincide.

Mechanical details cause their own share of trouble, beginning with improperly applied thermal interface material, whether too thick, too thin, or unevenly spread, which undermines an otherwise sound thermal path. Poor airflow planning starves downstream components of cooling air, and placing heat-sensitive parts near hot devices invites drift and premature failure, even when overall temperatures seem acceptable.

Aging effects are easy to overlook at design time. Fans wear and slow, interface materials dry out and degrade, and accumulated dust throttles airflow. A design with no thermal margin at release will face degraded thermal management over time in the field.

When these problems arise in the field, repairs can be costly in terms of downtime and resources. Consequently, real-world testing under controlled yet realistic worst-case conditions is vital for verifying thermal management. Verification begins during the design stage using multiphysics and CFD tools such as Ansys Icepak for advanced CFD modeling, COMSOL Multiphysics for modeling interactions across electrical and mechanical domains, SOLIDWORKS Flow Simulation for computer-aided design (CAD) integration, or Simcenter Flotherm for pc board thermal modeling.

Simulation tools help designers model heat sources, airflow, and temperature distribution under worst-case conditions before any hardware is built. Although simulation does not replace real-world thermal testing, it identifies layout and airflow problems while changes are still inexpensive. Used together, simulation and hardware testing can validate the static thermal design and identify critical problems before production and field deployment.

While this verification confirms the design’s performance under worst-case conditions, dedicated controllers provide the capabilities needed to respond dynamically to changes in thermal loads and operating conditions.

Controlling forced-air cooling with an intelligent fan controller

For designs that rely on forced air cooling, Analog Devices’ MAX31785AETL+T six-channel intelligent fan controller simplifies PWM and closed-loop revolutions per minute (RPM) fan control by using real-time temperature monitoring. It is well suited for equipment such as servers, communications infrastructure, and industrial power systems.

The device supports simultaneous control and monitoring of multiple fans via six independent PWM outputs and 12 tachometer inputs. For thermal monitoring, the controller accepts inputs from multiple sensors, including an internal temperature sensor and up to six remote thermal diodes. With these capabilities, designers can set fans to the lowest speed needed to maintain the desired temperature, reducing acoustic noise and fan wear. In the event of faults, the controller’s built-in fault detection raises alerts that help reduce downtime caused by failing fans or abnormal temperatures.

Capabilities such as fan speed control can operate automatically or via host commands. The MAX31785AETL+T provides a power management bus (PMBus) command interface to the host over a serial connection compatible with inter-integrated circuit (I²C) and system management bus (SMBus) signaling. For systems requiring dynamic airflow to ensure system reliability and minimize noise, the controller lets designers shape fan speed response to balance cooling needs against audible noise.

To accelerate evaluation and prototyping, Analog Devices’ MAX31785K evaluation kit (Figure 6) includes an assembled and tested board with the controller installed, a USB-to-I²C interface module, and a 12 V fan. The kit's USB connection requires no custom driver, and downloadable Windows software provides designers with register-level access.

Figure 6: The MAX31785K evaluation kit provides an assembled platform for exercising six-channel closed-loop fan control before committing to a system design. (Image source: Analog Devices)

Regulating precision cooling with a thermoelectric cooler controller

Forced-air cooling addresses many bulk cooling tasks, but some thermal loads require tighter temperature control than air movement alone can provide. Optical modules, laser diodes, and similar components maintain performance only within a narrow temperature window. For these applications, a TEC provides precise, component-level cooling under electronic control but brings its own design challenges. Analog Devices’ ADN8834WACPZ-R7 TEC controller (Figure 7) offers a patented single-inductor solution designed to deliver TEC cooling in space-constrained systems.

Figure 7: A TEC under closed-loop control holds temperature-critical loads such as laser diodes within a narrow thermal window. (Image source: Analog Devices)

Capable of delivering up to 1.5 amperes (A) of TEC drive current, the ADN8834WACPZ-R7 integrates proportional-integral-derivative (PID) compensation for accurate thermal regulation, and supports temperature monitoring with negative temperature coefficient (NTC) thermistors and positive temperature coefficient (PTC) resistive temperature detectors (RTDs). Its low-noise operation even makes it suitable for use in noise-sensitive analog systems. The ADN8834WACPZ-R7 can be evaluated using the ADN8834MB-EVALZ evaluation board (Figure 8) and associated daughtercards.

Figure 8: The ADN8834MB-EVALZ and associated daughtercards can be used to evaluate the ADN8834WACPZ-R7 TEC controller. (Image source: Analog Devices)

Conclusion

As power densities rise, the heat generated by switching transistors, diodes, magnetics, and capacitors threatens power supply efficiency, reliability, and component lifetimes. Designers can meet this challenge by quantifying each loss mechanism, selecting the cooling method that best fits the application, following proven thermal design practices, and using dedicated controllers to precisely regulate fans and thermoelectric coolers.