Tamper-Proof Power Supplies for Electricity Meters


Florian Mueller at Texas Instruments

Electricity meters use switch mode power supplies (SMPS) because of their high efficiency and good performance levels, it is a matter of common knowledge that SMPS rely on magnetic devices for their operation. A very popular and cost-effective topology used in modern electricity meters is a flyback, with a transformer essential for transferring energy and providing isolation from the primary to the secondary side.

 

Today’s most opportune criminals are tampering with electricity meters by using strong external magnetic fields to interrupt the operation of this power supply, illegally lowering their own electric bill. A common practice is to order a strong magnet – available from many online shops – which can be placed next to or on top of the transformer of the electricity meter power supply.

This magnet interrupts the entire operation and in some cases, can even destroy the power supply, rendering the unit inoperative. This happens when a strong external magnetic field causes the transformer core material to turn in similar directions, meaning the material saturates when it reaches its maximum flux density B.

Once saturated, the magnetic field strength is not directly proportional to the current flowing through the primary winding anymore. Therefore, the core loses its magnetic characteristics, the primary inductance drops, and the current increases, which can lead to a catastrophic failure.

This article presents some methods to make an offline flyback more robust in order to protect against a criminal employing an external magnetic field.

 

The Principle of Operation for a Valley Switching Flyback

The days when flyback controllers used a constant frequency are over. Typically, modern versions use a valley switching or quasi-resonant modulating technique to improve efficiency. This article will only consider a valley switching controller, where the switching frequency varies so the actual switching event happens on a valley of the switch node resonant ringing.

This particular controller modulates the switching frequency while simultaneously maintaining the primary peak current constant over most of the operating range. As the output load increases, the switching frequency increases, reaching a maximum switching frequency clamp (typical value = 100kHz-130kHz), which limits the maximum achievable output power.

 

The Role of Primary Inductance

Generally, the primary inductance (and turns ratio) of a flyback transformer will determine if the controller works in transition mode, deep in discontinuous mode or in continuous conduction mode for a full load.

The primary inductance between the primary and the secondary windings defines the operating frequency if a valley switching technique is used. An incorrectly calculated primary inductance can decrease efficiency or even make it impossible to deliver the full output power. The primary inductance, therefore, must be chosen with extreme care.

The challenge for creating a tamper-proof power supply is that the effective primary inductance decreases under the presence of a strong external magnetic field. However, what does that mean for the operation point?

If the controller is using a voltage mode technique, then a reduction of the effective primary inductance would suddenly lead to a high primary peak current, which forces the transformer deeper into saturation and can destroy the power supply. To solve this, a current mode controller (instead of a voltage mode controller) should be used, because it limits the primary peak current each switching cycle.

Normally, even under an external magnetic field, a current mode controller can keep the maximum primary peak current under control. If an external magnetic field is present, this controller will compensate for the lower primary inductance by increasing the actual switching frequency.

It is recommended to structure the primary inductance, so the switching frequency is way below the maximum switching frequency clamp of the controller. This means, it is recommended to choose a higher primary inductance, which will allow the controller to increase the switching frequency (during an external magnetic field attack), ensuring maximum power delivery.

Another advantage of a higher primary inductance is that if the external magnetic field ends up reducing the effective primary inductance, the ‘ON-time’ will not fall below the minimum ON-time of the controller, which could cause stability issues.

 

Preventing Fast Saturation

Equipping a transformer with a high number of primary turns reduces the magnetic flux ϕ. The reason for this is that a core with a higher magnetic reluctance Rm (due to larger air gap) must be used to maintain the same primary inductance. The flux density is, simply, the magnetic flux ϕ divided by the effective core area Ae. Therefore, a high number of primary turns (limiting flux ϕ) combined with a large effective core area Ae, ultimately reduces the flux density in the core. The inevitable tradeoffs, however, are a higher core and winding losses.

Probably the most effective way to prevent saturation is to use a core with soft saturation characteristics and a high saturation flux density, such as an iron powder core. This material will not saturate as abruptly as ferrite does. There are iron powder flyback cores available that can withstand a flux density of more than 1.5Tesla, which is a great choice for preventing saturation but, unfortunately, it comes at the expense of poor efficiency due to high core losses.

 

The Orientation of the Transformer

The orientation of the transformer plays a critical role beside of the core material, considering the transformer core provides an ideal path for the external magnetic field. The core greatly ‘amplifies’ the external field, and if the magnetic field lines of the external magnet have the same direction as the transformer flux density, the field inside the core can be very high. For this reason, a horizontally mounted transformer would be preferable to a vertical mounted one.

 

Tamper-Proof Reference Designs

A few tampering protected designs are available at TI’s website. For example, the reference design PMP30276 uses an iron powder core that can handle a strong external magnetic field, at the drawback of lower efficiency. The PMP30345 uses a Sendust core, which can also withstand a strong external magnetic field but isn’t as robust as an iron powder core.

Using a Sendust core is a good compromise between efficiency and tampering immunity, in which the PMP30435 uses the UCC28740 valley switching flyback controller from Texas Instruments. The optimized circuit and transformer make the design robust against an external magnetic field up to 200mT.

The test report below shows the measurement results with and without the presence of an external magnetic field. A magnet (Neodym, N35, Br=1.21T, 50mm x 12.5mm x 50mm) was placed on top of the transformer in order to generate a 200mT Field. The distance D between the transformer and the magnet was 5mm.

The figures show the behavior of the power supply when an external magnetic field is present. There are always two measurements, one with and one without a magnet placed on the top of the transformer.

Figure 1 shows the switch node voltage VSW (Drain-Source Voltage of primary MOSFET) without, and Figure 2 with an external 200mT field at full output power.

 

switch node voltage without an external 200mT field at full output power

Figure 1. Switch node voltage without an external 200mT field at full output power.

 

switch node voltage with an external 200mT field at full output power

Figure 2. switch node voltage with an external 200mT field at full output power.

 

It is evident the controller is operating with a switching frequency of 55kHz. When the external magnetic field is present, the controller increases the switching frequency to 85kHz. The maximum switching frequency of the UCC28740 controller is 100kHz. As mentioned earlier, the higher the switching frequency, the higher is the output power, which means there’s still some margin to deliver the maximum output power.

Figures 3 and 4 show the dynamic load regulation for a load step from 40% to 100% of the maximum output current.

 

loadstep 0.4Ato1.0A 0mT

Figure 3. Loadstep 0.4A to 1.0A 0mT

 

loadstep 0.4Ato1.0A 200mT

Figure 4. Loadstep 0.4A to 1.0A 200mT

 

Without an external magnetic field, the output voltage derivation is about 110mV. If a 200mT field is applied, the load step gets worse and the voltage derivation increases to about 220mV. This means the magnetic field reduces the bandwidth of the system.

A good sign is that the voltage of both measurements shows a damped ringing, an indicator of an adequate phase margin. This can be verified with the test report of the PMP30435 (available at www.ti.com) because it includes the measurement of the total open loop (small signal analyses). The bandwidth without an external magnetic field is 2kHz and the phase margin is 63°. If the magnet is placed on top of the transformer, the bandwidth reduces to 0.5kHz but with a sufficient phase margin of 70°. This means the system remains stable even under the 200mT external magnetic field.

 

Current Mode Controllers and Transformers Prevent Magnetic Tampering

If a tampering protected power supply is needed, then a current mode controller and a transformer with a high saturation flux density (e. g. iron powder) are recommended. A high number of primary turns, a core with a large effective core area, and a horizontally mounted transformer will reduce the maximum flux density inside the core.

The primary inductance should be specified carefully in order to achieve a stable system, even under the presence of a strong external magnetic field.

 

About the Author

Florian Mueller was born in Rosenheim, Germany, in 1976. He received a degree in electrical engineering from the University of Haag. After working for several years as a freelancer in the field of electrical engineering, he joined TI in 2008 and is working in the European Power Design Services Group, based in Freising, Germany. His design activity includes isolated and non-isolated DC/DC and AC/DC converters for all application segments.

 

More information: Texas Instruments    Source: Bodo's Power Systems, October 2018